The Ant-Man star was thrilled when Dax Shepard picked him to play the role Erik Estrada created on TV, and thought he'd be a "bad-a**" in the saddle as he started preparing to play a California Highway Patrol cop - but he was sadly mistaken.

He explains, "I had to do motorcycle training... When you get on, you're like, 'Oh man, I got this' and I had somebody, like, video me and it's the slowest riding that you possibly have.

"I've got a kid so I'm like, 'Man, I could die on this!'

"I sent the video to Dax and he was like, 'I'm not impressed'."

Pena, whose brother is a real-life police officer, is also on a strict diet for the role, so he can pass as a hunky bike cop.

"I'm, like, eating less than 2,000 calories," he explains. "I was slightly overweight... I'm eating, like, just grass and rocks... It's been going on for, like, two-and-a-half months. I dropped, like, 20 pounds - I had some good cushion. I was never cold."

He has also hired Channing Tatum's fitness guru to help him reach his goal before filming begins.

The actor adds, "If I try to puss out... and say, 'I think we're good, dude, I just threw up a couple of times. I think we should call it', (he says), 'Channing wouldn't call it!'"