These actors and actresses take their jobs seriously, and for these big roles that meant extreme weight loss or packing on the pounds. The transformations are jaw-dropping.
MORE: 13 Celebrities Who Almost NEVER Smile in Photos

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Celeb Weight Loss Weight Gain Anne Hathaway
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Anne Hathaway in Les Miserables
The actress lost a reported 25 pounds for the role by eating only an oatmeal paste twice a day.
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Celeb Weight Loss Weight Gain 50 Cent
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50 Cent in All Things Fall Apart
In just 9 weeks, this actor/rapper dropped from 214 to 160 pounds by living on a liquid diet.
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Celeb Weight Loss Weight Gain Jake Gyllenhaal 1
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Jake Gyllenhaal in Nightcrawler
You can clearly see the almost 20 pound drop in this actor's gaunt face.
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Celeb Weight Loss Weight Gain Jake Gyllenhaal 2
Jake Gyllenhaal in Southpaw
After Nightcrawler, this actor turned around and packed his newly lean frame with muscle.
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Celeb Weight Loss Weight Gain Renee Zellweger
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Renee Zellweger in Bridget Jones's Diary
One of the most famous Hollywood transformations, the actress gained 30 pounds for this role.
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Celeb Weight Loss Weight Gain Christian Bale 1
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Christian Bale in The Machinist
Easily the most shocking transformation was this actor's drop to a dangerous 121 pounds.
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Celeb Weight Loss Weight Gain Christian Bale 2
Christian Bale in Batman Begins
This actor turned right around from dropping to 121 pounds to bulking up to a muscular 190.
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Celeb Weight Loss Weight Gain Charlize Theron
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Charlize Theron
She packed on 30 pounds for this critically acclaimed role.
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Celeb Weight Loss Weight Gain Jared Leto
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Jared Leto in Chapter 27
The singer/actor gained 67 pounds to play John Lennon's murderer.
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Celeb Weight Loss Weight Gain Natalie Portman
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Natalie Portman in Black Swan
The already-trim actress leaned out even more for her role as a ballerina.
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Celeb Weight Loss Weight Gain Mila Kunis
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Mila Kunis in Black Swan
The That 70's Show actress slimmed down to a scary 95 pounds for the film.
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Celeb Weight Loss Weight Gain Matthew Mcconaughey
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Matthew McConaughey in Dallas Buyer's Club
The actor dropped 38 pounds to a skinny 135 to play a man with AIDS.
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Celeb Weight Loss Weight Gain Tom Hanks
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Tom Hanks in Cast Away
To play a man stranded on an island, this actor reportedly dropped 50 pounds.
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Celeb Weight Loss Weight Gain Matthew Fox
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Matthew Fox in Alex Cross
The 40 pound drop this actor went through is apparent in his lean face.