If you saw Jonah Hill in his Oscar-nominated role in "Moneyball" - or on a talk show lately - it's hard to believe he's the same actor who played the overweight, horny, wisecracking teenager in "Knocked Up" and "Superbad."

Yes, some time has passed since those movies, but mainly Hill has reinvented himself. Or, as he told Bill Weir in an interview for "Nightline," he grew up.

"I'm almost 30 years old - I'm not a foul-mouthed seventeen-year-old kid who's trying to get laid. … I'm an adult man who takes his work really seriously," said Hill. "I'm an actor, I'm not a comedian, I never was a comedian."

After "Knocked Up," "Forgetting Sarah Marshall" and "Get Him to the Greek," Hill started declining comic parts, a risk that paid off in his well-reviewed turn as Yale-educated statistics nerd Peter Brand in "Moneyball."

Along with growing up has come slimming down.

"It was just mostly diet," said Hill. "I wish there was some crazy thing that I did, like a pill or a genie or something, but it was, unfortunately, I went to see a nutritionist, and he told me what to eat to change my habits and stuff… I found that Japanese food was very helpful to me."