Image courtesy of NBC Universal

“Knocked Up is about a guy [Ben], played by Seth Rogen, who is starting a website, is kind of a slacker, who goes out to a bar one night and is lucky enough to end up back home with Katherine Heigl’s character [Alison], and what should have been nothing but a one night stand, next thing you know she’s knocked up. So it’s the story of the two of them coming to terms with the fact that they’re sorta stuck together,” says actor Jason Segel (How I Met Your Mother), who, along with Jonah Hill, Jay Baruchel and Martin Starr, plays one of Ben’s four roommates in this summer’s Knocked Up.

Jonah Hill offers a somewhat different take on the movie. “The plot is basically like Letters from Iwo Jima. We actually ripped off the same plot, and we just added a few ‘fucks’ every now and then,” he jokes, when asked whether there is more to the story than the title and movie trailer suggest. But Jason and Jonah (whose characters also happen to be named Jason and Jonah) do agree that what makes Knocked Up work so well is largely writer/director Judd Apatow’s approach to movie-making. Both have worked with Apatow before (Jonah had a small but memorable role in The 40-Year-Old Virgin, and Jason starred in Apatow’s television cult classic Freaks and Geeks, among other things) and credit the success of his films with the fact that he trusts his actors enough to let them improvise.

“We spend a lot of time rehearsing and improvising and then the script takes the shape of the direction the improv took. It’s great because by the time you arrive on set, nothing that’s coming out of your mouth feels forced; it’s all sort of in your words. And that’s part of what Judd recognizes – that the acting turns out best when the actors are comfortable,” Jason says, adding, “Anytime Judd comes calling, I’m there. Judd is my mentor and sorta my professional idol as well.”

Instances of spontaneous humor aren’t necessarily the only benefits of the improv approach to a movie like Knocked Up. “Even in the more intense scenes, Seth and Katherine did a lot of improv, and it makes the fights seem more real. That’s how I like to work, and Judd is the best at that in my opinion. I think the best stuff comes from being in the moment, just off the cuff,” says Jonah, echoing the sentiments of his co-star.

Most people would agree that waking up from an alcohol-induced sleep coma in the bed of a girl who looks like Katherine Heigl (or even someone half as hot) is pretty sweet, even if you can’t technically remember her name or how you got there. And there are only two things that might ruin that feeling of self-satisfaction: one would be discovering that you not only had the best night of your life, but you also have the clap; the other would be learning that the girl is pregnant with your seed. So what exactly do Ben’s roommates have to say when they find out about his alcohol-induced, life-altering one-night stand? “We’re basically the devil in his ear. We’re kind of giving him advice that’s crazy. Obviously we’re all ill-prepared to handle one of our best friends having a child, and I think we’re even more immature than he is, so I think we give him terrible advice, [we] just don’t know how to handle it,” says Jonah, 23. “It really freaks us out, and also the big inspiration for me at least was, I was just picturing that this was Seth in real life since I’m friends with him. In real life, he’s obviously way more mature than his character, but it would freak me out if one my best friends was having a kid because I would feel like things wouldn’t be the same, and I would kind of feel like our friendship wouldn’t be the same… it would just be super frightening.”

Super frightening indeed, but if Knocked Up the movie is any indication of the real-life experience, then take solace in knowing that if one of your drunken, idiot friends accidently impregnates a hot girl he met at a bar and they decide to have the kid together, it can also be super hilarious!