"And I really didn't think I was going to get it," says Hill today. "because, you know, movie people are way over there, and I'm just this chubby New York Jewish kid who really likes movies. But, I got the part. And then there was that scene in The 40 Year Old Virgin ... "
Ah, yes, Hill playing the awkward but determined customer who doesn't understand the rules of an eBay store, where he can hold the object he wants -- in this case, fine platform boots with goldfish bowls as heels -- but he can't pay for them there and then. He has got to go online, pay for them and have them shipped out. The scene was supposed to last no more than a line or two, but Hill was encouraged by director Judd Apatow to improvise. And the scene just grew and grew.
When The 40 Year Old Virgin opened, Jonah Hill landed firmly on Hollywood's map. And now, thanks to the likes of Superbad (2007), Cyrus, Get Him To The Greek (both 2010) and last year's Moneyball (for which Hill received a Best Supporting Actor Oscar nomination), our boy is, well, a Hollywood player, writing, producing, and creating actual hits.
Following on from his Simpsons-inspired animated sitcom Allen Gregory (which he co-created, wrote, executive produced and starred in) comes 21 Jump Street, a big-screen adaptation of the 1980s US TV series that made Johnny Depp a pin-up.
Hill came up with the story, executive produces once more, and takes the lead alongside hunk-of-the-month Channing Tatum as a pair of incompetent, babyfaced rookie cops who go undercover at an LA high school to try and find the source of a new drug sweeping the nation.
"In America, the show was a very big deal," says Hill. "And it was such a neat idea for a comedy, I always had it in the back of my mind that someone should try and bring it to the big screen one day. I never thought it would be me. Well, I was always secretly hoping it would be me, of course."
That 21 Jump Street is already garnering some rave reviews in the US is a pleasant surprise for Hill.
"Comedies tend to get a rough ride from critics," he says. "As do big-screen adaptations of small-screen hits, so, you know, the odds were stacked against us. But we went in there determined to have as much fun as possible."
The original series gave the future box-office giant Depp his first taste of stardom, and he didn't like it much, walking out on 21 Jump Street before his contract was up. For Hill, getting the prodigal son on board was always a priority.
"It's like stepping into the Star Wars world and trying to avoid Darth Vadar," he smiles. "It was pretty crucial to me that Johnny came on board for a cameo. Once he agreed, it was a case of finding a funny way to do it. I reckon most people will be hard-pressed to spot him until he steps forward.
"Johnny's a great sport and always up for a laugh, so, he was pretty much game for anything. In the end, he felt it was cathartic too, a chance to say sorry, in a way, to his co-stars for bailing on them."
With the Oscar nomination for Moneyball under his belt, and the Israel Film Festival bestowing their Achievement In Film Award on him this month, Jonah Hill is now trying to get his head around the fact that he's got power in Hollywood these days.
"The teensiest, teensiest amount of power," he smiles, "but, I realise now, with 21 Jump Street, that I can actually help get a movie made. That's kind of wild, because I never really saw myself in that role. Everyone dreams of getting to a point where you can call the shots, or, at least, have a voice in the decision-making, so, life is pretty sweet right now. Having this little bit of power, it's great; I feel like mafia."
Not that Hill will be able to play the heavy anymore, having been inspired during the making of 21 Jump Street to lose some weight. In the end, he lost more than 30 pounds. It was a visit to a nutritionist in New York that prompted a diet change, Hill switching to sushi to lose the weight.
"It was important to me, not just because of my health, but also, as an actor, you want to be able to cover a range of roles," he explains. "Being a chubby guy, I realised I was probably going to keep getting chubby guy roles. And they're usually comedic roles too. So, the weight loss will hopefully change that.
"I want to make more movies like Moneyball, right alongside movies like 21 Jump Street. I want to prove to the world that you don't have to be fat to be funny. It helps, of course, but it's not crucial ... "
21 Jump Street hits Irish screens on Friday