Zac Efron
Efron has always been in pretty good shape, for what it's worth, as an actor. But, when he made his transition from Disney star into an adult star, he bulked up a little bit to play sex-appeal roles. Then, he got cast to play Brody in Baywatch...
Big Man on Campus
While the end result was great, Zac Efron hated the workouts. He felt like he was pushing himself further than he had ever pushed himself. Which, admittedly wasn't bad in and of itself, but he didn't like the long-lasting negative effects that came afterward. He was constantly dehydrated, in immense pain, and extremely tense due to his diet and schedule.