When Justin Long learned he would be doing full-frontal nudity in Going the Distance, he never imagined he'd be doing it in front of his childhood crush, Christina Applegate.

"I was sort of exposing myself to someone I grew up watching. Really weird," the actor, 32, told at the film's London premiere Friday. "And while we were shooting! I'm kidding."

The actor admitted that he isn't used to having so much attention on his body.

"There was an element of vanity that came into play that I wasn't really used to with the other roles I have done," he said.

So how did Long prep for the nerve-wracking scene?

"I had a very complicated procedure: nipple enlargement surgery. I did!" he joked. "I had very tiny nipples. They're like freckles."

Kidding aside, Long said he had a blast filming the movie with Drew Barrymore (who he's been dating off-and-on for several years) -- and even began to see her in a new light.

"She's so funny, insanely funny. But I also got to see a side of her during this movie, which I hadn't seen, because I'd never known her in a professional capacity. I got to see how she treated the people that she's working with and it made her so much more beautiful," Long told Us. "She's lovely  to everyone -- to PA's, studio people, no matter where you are in the hierarchy. It's rare."