Zac Efron is know for his lean muscular physique.

The 5'8" heartthrob usually weighs in around 145 pounds, but he gained over 18 pounds to play a Marine in the upcoming film "The Lucky One."

To put on the weight, Zac was put on a 3,500 calorie a day diet and underwent months of intense training, including working out five days a week (with lots of powerlifting) and drills in a full Marine kit.

"By the end of the movie I didn’t recognize myself," the actor told Men's Health. "You hear about guys like Christian Bale who dive into it and are really able to transform. I’ve always wondered if I had the willpower to actually do it. And I’ll always have pride around the sense that I can."

For more photos of Zac, Christian and other actors who dramatically changed their bodies for roles, click on the "Launch Gallery" button below.

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"The Lucky One" opens in theaters this Friday, April 20.
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