Alex Breaux faced a twofold challenge in portraying Ray, a champion swimmer in the dark and twisty “Red Speedo” at New York Theater Workshop. He had to look convincing as an aspiring Olympian who spends the whole show in the swimsuit of the title. And he needed to, well, play dumb — at least outside the water, where his character is caught up in a doping scandal that could derail a lucrative future.

He applied his background as an athlete to both.

Six-foot-three and slim, Mr. Breaux, a 30-year-old former Harvard University wide receiver, was in good shape when he was cast in Lucas Hnath’s play last fall — he has taught spinning seven mornings a week since he was in graduate school at Juilliard — but he said he didn’t want audience members to have to suspend any disbelief.

So he undertook a grueling exercise and diet regimen that has dramatically reduced his body fat while giving him an extremely defined chest, back and washboard abs. He acknowledged losing more weight than he intended — he wouldn’t say what he weighs now — but said he feels satisfied with the look he has achieved for the role.

As for why Ray seems dense, Mr. Breaux noted that athletes who have to do a very repetitive movement over long periods — running or rowing or swimming — need to be able to get into a Zen-like state.