For his new series, Pablo Schreiber had to bulk up to 230 in order to play a retired boxer who manages his brother, a former heavyweight champion. What's more, the six-foot-one actor had to get large without too much muscle definition. Schreiber's next part was playing a clumsy, skinny kid in the play Gruesome Playground Injuries, so he set about shedding 30 pounds, to his normal weight of 200.

To slim down and get ripped, Schreiber, 32, didn't turn to a trainer or a gym; he turned on his TV—in the form of the popular workout DVD series P90X. Developed by Tony Horton, the at-home exercise program emphasizes "muscle confusion," putting people through different strength and cardio exercises. "It's definitely the backbone of my routine," says Schreiber, who also stars in the film Happythankyoumoreplease but is best known for playing Nick Sobotka in HBO's The Wire (and for being Liev Schreiber's half-brother). "The whole plyometric DVD is just a freaking killer."

The Secret Weapon: The Slackline

Schreiber is a devotee of slack lining—it's like walking a tightrope, but the line has more give (hence the name), making it harder to balance. The actor practices on it several times a week during the summer—mostly walking, doing squats, and performing yoga moves. "Just the act of balancing is incredibly difficult," he says. "The first time you try it, you think you'll never be able to do it. Your legs just start going into convulsions and shake all over the place."

Ideal for beginners, The eLine is wide and firm, meaning it won't snap in half easily. ($90; YogaSlackers.com)

Get His Workout

P90X comes with 12 DVDs—focusing on different body parts for weight workouts, as well as cardio, core, kenpo, and yoga. To complete the hour-a-day home workouts, you just need a set of dumbbells (or resistance bands) and a pull-up bar, all of which can be purchased for about $170. (beachbody.com)

"'P90X is all jump training. You squat-jump and turn 180 degrees back and forth. You do guitar-hero hops, like you're Pete Townshend, kicking your ass with your heels. It fucking hurts—I won't lie to you." — Pablo Schreiber

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