For some actors transforming into their characters requires a lot of commitment.
But some have taken it to extremes over the years by either gaining or losing weight as quickly as possible, regardless of the detriment this might have to their health.
Here are some of the actors to have the most dramatic transformations onscreen.
Charlize Theron famously went through a major transformation to play real life killer Aileen Wuornos, she shaved her eyebrows and piled on 30lbs by eating doughnuts and crisps, she'd even keep a "secret stash" with her at all times to help with the weight gain.
Read more: Charlize Theron Says She’ll Never ‘Gain 40 Pounds’ for a Role Again (IndieWire, 2-min read)
But the South African actor recently admitted she would "never" make a film again where she was required to do so following her experiences with Monster and Tully.
In an interview with Allure, the actor said: "When I was 27, I did Monster. I lost 30 pounds, like, overnight. I missed three meals and I was back to my normal weight.
“Then I did it at 43 for Tully, and I remember a year into trying to lose the weight, I called my doctor and I said, ‘I think I’m dying because I cannot lose this weight.’ And he was like, ‘You’re over 40. Calm down. Your metabolism is not what it was.’ Nobody wants to hear that.”
Christian Bale’s ability to transform his body for movie roles is legendary, and his size has yo-yoed dramatically over the years depending on the role he's taken on.
For American Hustle the actor put on 43 lbs (3 stone) to play conman Irving Rosenfeld, and the weight gain led him getting a herniated disc according to director Dave Russell.
His most famous transformation was of course for The Machinist, for which he dropped 60lbs (over 4 stone) and appeared positively skeletal in the pyschological thriller.
Chris Hemsworth may be best known for his impressive physique in the Thor movies but his most drastic transformation came for the film In the Heart of the Sea.
Playing a sailor who was lost at sea, Hemsworth slimmed down by eating just 500 calories a day, and he told EW at the time of the film's release how difficult it was to do.
Hemsworth explained: "When you're already starting off lean, its brutal to chew through that kind of weight. Every pound feels like a kilogram."
Jared Leto is another actor who has famously lost and gained a lot of weight for different roles, he is also known for going to extremes with his approach to method acting.
In order to gain the 67 lbs (over 4 stone) to play John Lennon’s killer Mark Chapman in Chapter 27, Leto guzzled pints of microwaved ice cream mixed with soy sauce and olive oil. The intense weight gain gave the actor gout.
For Dallas Buyers Club Leto played Rayon, a role that would lead to his first Oscar win, and he dropped down to just 114 lbs (8 stone) by stopping eating entirely.
Similarly to Leto, his co-star Matthew McConaughey also lost a significant amount of weight for his role as Ron Woodroof, a man diagnosed with HIV, which earned him a Best Actor Oscar.
The actor slimmed down to 133 lbs (9.5 stone) for the role with a diet of Diet Coke, egg whites and a piece of chicken a day, although it had a detrimental affect on his health.
McConaughey said of the extreme weight loss: “As soon as I hit 143lb [10 stone 3lb], I started losing my eyesight.”
In order to get a sculpted figure for Baywatch Zac Efron was pushed to his "functional limits" by his trainer, who he trained with four to five times a week whilst following a strict, clean diet.
He may have looked good on camera but it took Efron six months to recover from the experience, and he developed depression and insomnia because of it.
Efron told Men's Health: "I started to develop insomnia, and I fell into a pretty bad depression, for a long time. Something about that experience burned me out.
"I had a really hard time recentering. Ultimately they chalked it up to taking way too many diuretics for way too long, and it messed something up.”
In order to star in Stanley Kubrick's Full Metal Jacket as Private Pyle, Vincent D'Onofrio gained 70lbs (5 stone) over 7 months through a diet of greasy food.
In an interview with the Chicago Tribune, he said: “I gained weight everywhere. My thighs were tremendous, my arms were tremendous, even my nose was fat."
The transformation led to a knee injury, but it also impacted his life in other ways: “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.”
In order to play hunger striking IRA man Bobby Sands, Michael Fassbender dropped 42lbs (2 and half stone) from his already slender frame for Steve McQueen’s acclaimed drama.
Jake Gyllenhaal felt that Nightcrawler's oddball freelance journalist who was hungry for success so he should look hungry for real and dropped 30 lbs through a calorie-restricted diet.
He achieved this by tricking himself into thinking he was eating when he wasn't, with his co-star Riz Ahmed saying: “He’d have a bowl of luxury chewing (gum) and they were, like, really elaborately flavored, to trick his brain into thinking he was having a meal.
"So he was on his exercise bike having chewing gum and, like, almond tea with one calorie."
When rapper 50 Cent shared an image of himself for the film Things Fall Apart people were shocked by his gaunt appearance.
The musician had lost 4 stone in 9 weeks through a strict liquid diet and gruelling gym sessions to play someone with cancer, but ultimately the film wasn't given a wide release.
Watch the trailer for Monster: