“Bruce didn’t have his prom last week,” says Gunnar Peterson with a grin.

Still, says the trainer who helped whip Bruce Willis, 52, into shape for Live Free or Die Hard, “you don’t see many in his age bracket knocking it out at the gym with his dedication.”

To get back into fighting form after a 12-year Die Hard hiatus, Willis did hour-long cardio and strength sessions nearly every day for three months before filming. On set he worked out during lunch. Peterson was impressed:

“His thing is being able to do what the role requires.” In this case that meant having a body that looked credible hanging from moving vehicles, dodging bullets and trotting around in an ever-more-torn shirt.

Though you might guess that once stars hear “that’s a wrap,” they exhale and head for the sofa, Peterson says Willis kept up his routine. “What if the director decides to reshoot?” Eventually, Willis may let himself go a little. “It’s not natural to stay completely shredded year-round.” [via]