Actors aren’t afraid to completely transform themselves for a role. With the help of nutritionists and trainers, they’re able to pack on the pounds in order to fully immerse themselves in a part. Gaining a potbelly or a pair of man boobs might be necessary for a certain character in the same way that shedding a ton of weight might fully actualize another.

However, gaining weight doesn’t necessarily mean getting fat. In fact, many of the actors on this list went through the strenuous journey of accumulating pure muscle, using intense dieting and hardcore workouts for months at a time to get into the shape of their lives. Other times, of course, there’s the fun kind of weight gain—the kind that involves indulging in loads of junk food and sitting on your butt!

Of course, packing on the pounds for a role isn’t a guarantee of a good film. In fact, some of the most transformative actors we know blew themselves up–and jeopardized their health in the process–only to have their film become a major failure (looking at you, Jared Leto). These 15 actors, however, know exactly how much poundage it takes for a role to pay off!

Renee Zellweger

We’re used to seeing actress Renee Zellweger looking svelte on the red carpet, but her biggest role to date–as Bridget Jones in the Bridget Jones trilogy–saw the actress pile on weight in a move that softened her features, endeared her to audiences, and saw her move from playing just a love interest to a leading lady.

For the first two films, Zellweger put on 30 pounds by following a 4,000-calorie-a-day diet (she was not allowed to gain weight for the third installment, on the advice of professionals). In between both movies, Zellweger dropped the weight and then some for the role of Roxie Hart in the Oscar-winning Chicago, which caused many to speculate about the actress’ yo-yo dieting and its effect on her health. Ballooning from a tiny size 4 to a 14 may have stressed the actress both physically and mentally, but the first two films ended up grossing over $544 million worldwide!

Tom Hardy (Bronson, Batman)

It’s hard to think that there was ever a time when Tom Hardy wasn’t his beefed up sexy self, but it’s true! The Brit actually used to be on the smaller side before he gained international recognition for his role as Bane in The Dark Knight Rises. However, before the 2012 film, Hardy had already undergone a drastic physical transformation for the movie Bronson, which was released in 2008. To play the man who has been dubbed “Britain’s most violent prisoner,” Hardy added 45 pounds of muscle and fat to his naturally lean frame, gorging on pizza and ice cream at night and indulging in only chicken and rice during the day.

For The Dark Knight Rises, Hardy had to again gain weight. But this time, it was 30 pounds of pure muscle. While fans wanted the actor to add another 30 to that (bringing his total weight to 220 lbs), Hardy defended the work he had done, and we think that the physical evidence spoke for itself!

Ryan Reynolds

Another actor who we’re used to seeing in all his rippling, muscular glory, Ryan Reynolds wasn’t always the buff guy we see from The Green Lantern and Deadpool. In fact, you could argue that he used to be downright skinny! Back in 2004, Reynolds was slim before working out for three months in order to add 25 pounds of muscle to his frame for Blade: Trinity. Crediting a 3,200-calorie-a-day diet and six days a week spent working out to get his ripped physique, Reynolds’ trainer also offered some insight into how they sculpt the actor’s body depending on the role.

“For Blade and Green Lantern,” his trainer, Bobby Storm, told WebMD, “He was 200 pounds and 8% body fat. For rom-coms, he’s about 180 pounds and 11% body fat.” Either way, we can’t deny that Reynolds is drop-dead gorgeous and is proof of what a little added muscle can do for a man’s physique.

Jake Gyllenhaal

Jake Gyllenhaal is no stranger to transforming his body for a part. In 2010, the actor bulked up in order to play the role of a video game character in the colossal flop, Prince of Persia: The Sands of Time. Working out for six days a week for four months allowed Gyllenhaal to sculpt his body into the shape needed. But unfortunately, the film didn’t do nearly as well as the time he put into preparing for it.

Then, in 2014, Gyllenhaal appeared in the thriller Nightcrawler as a petty thief and cameraman. The role required Gyllenhaal to drop 30 pounds, something the actor said was vital to the role since the character was always hungry—both literally and figuratively. That paid off, earning Gyllenhaal multiple award nominations, including one for a Golden Globe. Shortly afterwards, he was required to undergo yet another transformation, this time for a part in the film Southpaw. To play boxer Billy Hope, Gyllenhaal regained the 30 he lost and added 15 pounds more of muscle using a high-protein diet.

Christian Bale

If anyone is a true master in transforming his/her body for a role, it has to be Christian Bale. The Academy Award winner has whittled himself down to 122 lbs for his role in The Machinist, bulked up to a muscular 200 lbs for Batman (he initially was 220, then lost 20 after being declared “too big”), lost 30 lbs to play an addict in The Fighter, and then gained about 60 lbs for 2013’s American Hustle.

Oh, and in between a couple of those, he also had the Batman sequels, so we’re a little shocked that the actor’s internal organs haven’t called for a ceasefire yet! Beefing himself up to 228 lbs to play the potbellied character in American Hustle, Bale also shaved his head and slouched so much that he ended up herniating a disc in his back! His transformation was so complete that costar Robert De Niro had to ask him who he was once the two were on set! The film received ten Academy Award nominations, and although it didn’t win any, we think it’s safe to say that this was definitely a success.

Jennifer Aniston

Jennifer Aniston is synonymous with California cool, natural tans, yoga, and Smartwater, so it’s a bit of a shock to see the actress looking anything but easy, breezy, and beautiful. However, Aniston has also long been on a quest to be taken seriously as an actress outside of the sitcom world that made her famous. One of the most recent examples of this is for her role in the 2014 film, Cake.

In the movie, Aniston played a grieving woman, and since diet and exercise were likely to be the last things on the character’s mind, she simply stopped going to the gym for a couple months and was less rigorous about her diet. The role was also notable for requiring the famously bronzed actress to go without makeup.

While the film received mixed reviews, many praised Aniston’s performance, and she earned a Golden Globe nod for her efforts. It may not have been the breakthrough she was hoping for, but it was a good start and worthy of being on this list!

Will Smith

Since seeing him in films like I Am Legend and other roles that allowed us to drool over his ab-baring torso, we might need some reminding to recall that when Will Smith came onto the Hollywood scene, he was just a lanky dude with a penchant for one-strapped overalls. On The Fresh Prince of Bel-Air, Smith was a tall, skinny kid who had a way with the ladies. But by 2001, he was proving that the boy was gone and a man was in his place.

For the role of Muhammad Ali in 2001’s Ali, Smith bulked up by 35 pounds of pure muscle to play the recently deceased boxer. Unlike other major movie stars who require a few months in order to get into fighting shape, the lean actor required almost an entire year of intense workouts in order to get his body up to snuff, especially since many critics were skeptical that he could pull it off. Well, pull it off he did! Smith was nominated for multiple awards for the part, including an Academy Award nomination for Best Actor.

Michael B. Jordan

Another boxer on the list (that’s three so far, if you’re keeping track at home), this time it was a fictional character that got Michael B. Jordan to transform into the best shape of his life. As Adonis Johnson in the 2015 film Creed (another film in the Rocky franchise), Jordan had to do his fictional father proud, and to do so required a helluva lot of work!

The young actor was in pretty good shape to begin with, but as a ripped boxer who was taken under the wing of Sylvester Stallone, Jordan began working out hardcore, to the point where he “felt muscles I [he] never knew [he] had.” Jordan ended up packing on 24 pounds of muscle through an intense workout regimen that incorporated boxing, as well as other weight training moves, and a diet of six protein-packed meals per day. Creed was nominated for a slew of awards, but was unfortunately snubbed at the Oscars, save for one lone nomination for Stallone.

Vincent D’Onofrio

Few actors are honest about how gaining weight for a film makes them feel, except for the occasional mention of how it destroyed them physically, or had lasting effects, like Tom Hanks, whose yo-yoing led to him being diagnosed with diabetes. Actor Vincent D’Onofrio, however, was outspoken about how he felt emotionally after gaining 70 pounds for his role as a troubled Marine Corps recruit in Full Metal Jacket.

“It changed my life,” he said. “Women didn’t look at me; most of the time I was looking at their backs as they were running away. People used to say things to me twice because they thought I was stupid.” D’Onofrio came close to weighing 300 lbs, tipping the scale at 280. The film is considered a major achievement by the multi-talented Stanley Kubrick, and despite his transformation from a trim athlete to a balding, unhinged private, D’Onofrio would pack on the pounds again in 2002 for The Salton Sea.

Hilary Swank

Hilary Swank whittled herself down for her role as Brandon Teena in Boys Don’t Cry–a role which won her an Academy Award–before she later bulked up to play a boxer in Million Dollar Baby, a role that earned her her second Oscar!

Swank trained for 90 days and consumed 210 grams of protein each day to get into fighting shape and put on 19 pounds of muscle in total. Despite winning the gold statuette, however, Swank wasn’t shy about admitting that the ordeal was horrible saying, “My training was two and a half hours of boxing and approximately an hour and a half to two hours lifting weights every day, six days a week…I would drink my egg whites because I could never eat eight to twelve egg whites in a sitting. It was just the worst thing ever.” Although the actress was only required to put on ten pounds for the part, she added another nine and showed that the extra meat on her bones gave her that much more power to win over the critics.

George Clooney

George Clooney is the timeless kind of handsome that has put him on the most beautiful lists for decades. With his salt and pepper hair and disarming smile, Clooney has made women of all ages swoon, even when he had a little more of him to love. To play a CIA operative in 2005’s Syriana, Clooney grew out his beard and packed on 30 extra pounds–something Clooney did NOT enjoy.

While we might take pleasure in the occasional junk food binge, an active actor like Clooney hated piling on the weight. According to friend Matt Damon, Clooney was really depressed during filming since he wasn’t allowed to move around in order to take the weight off until after production. Clooney admitted the same, stating, “There was nothing fun about it. There was not a moment of fun about shooting this film.” Fortunately for him, the extra weight helped nab him the Academy Award for Best Supporting Actor!

Morgan Spurlock

We know that Morgan Spurlock isn’t an actor, but as the central figure in his film Super Size Me, we think it makes sense to include him on this list, since gaining weight for the movie arguably created his entire career! The documentary–which sought to see what the effects on the human body would be after consuming a diet consisting only of McDonald’s for 30 days–launched Spurlock into the public eye and actually spurred the removal of super-sized products from the fast food chain.

Throughout the course of filming, Spurlock’s McDonald’s-only diet saw him gain 24 pounds over the month, experience mood swings and s*xual dysfunction, raise his cholesterol, and build up fat in his liver. While it only took the filmmaker 30 days to gain the weight, it took 14 months to lose it all, using a vegan diet coordinated by his then-girlfriend. Since its 2004 release, the film has been nominated, has won multiple awards, and has been viewed regularly in most health classes.

Charlize Theron

Naturally, no list would be complete without including Charlize Theron! The South African actress was known for her leggy blonde beauty before she took on the role of serial killer and prostitute Aileen Wuornos in 2003’s Monster. In fact, the transformation was so drastic that many critics and audience members were worried about how different the gorgeous actress looked and took stock of all her physical changes. She shaved her eyebrows! She was wearing false teeth! She had makeup applied that made her skin look rough! And, worst of all, she gained weight!

Theron added 30 extra pounds to her frame–something she said then-boyfriend Stuart Townsend loved–by eating Krispy Kreme doughnuts, chips, and all manner of cheesy things. Despite the controversy, Theron said that it wasn’t simply about getting fat. It was that she was trying to understand Wuornos as a person, including all the pain she carried with her. The performance won Theron a Best Actress Oscar, and she’s recently done it again. This time 35 lbs for the upcoming film Tully.

Gerard Butler

Packing on the pounds for a role doesn’t necessarily mean getting fat! Gerard Butler had a normal body prior (and some times a dad bod afterwards) to the filming of 300. Be honest, do you remember much about the special-effects-filled film besides Butler’s ripped abs? We didn’t think so.

Back in 2006, Butler prepped for the physically intense role by hiring three separate trainers to whip his body into gear for the role of the Spartan. With the help of a workout regime called the “300-Rep Spartan Workout” and the aid of a bodybuilding coach, Butler was able to add some considerable muscle to his frame. While we understand that his regime required some cutting before he added on the bulk, the role was easily the most recognizable of his career to date, and it catapulted him into action hero stardom that saw him go on to play the lead in films like Olympus Has Fallen, Gods of Egypt, and London Has Fallen (hey, we didn’t say they were GOOD movies).

Robert De Niro

Now that’s what we call dedication! For only one scene in the 1980’s movie Raging Bull, Robert De Niro gained an astonishing 60 pounds!

Yep, you read that right. 60 pounds for one scene!

To play boxer Jake LaMotta, De Niro got into great shape for all the fighting scenes, even going under the tutelage of the real-life LaMotta and participating in three boxing matches in order to be as realistic as possible. Then, director Martin Scorsese shut down production for four months so that his star actor could go on a binge-eating tour of Europe, specifically Italy and France. Over those four months, De Niro ballooned up to 215 pounds to play the “Old Jake LaMotta.” While eating through Europe was undoubtedly a good time for De Niro, the short-term weight gain was rough on his system, causing the actor to tire quickly and become labored in his breathing. The suffering was worth it, though, as De Niro’s transformation raised the bar for actors everywhere and earned the New York native both a Golden Globe and an Oscar!

Sources: dailymail, shape, thisisinsider