Knocked Up Judd Apatow Seth Rogen Katherine Heigl
Seth Rogen and Katherine Heigl in Knocked Up. Credit: Universal Pictures

A random conversation between Judd Apatow and Seth Rogen inspired the 2007 rom-com Knocked Up, according to director Judd Apatow. The movie turned 15 years old on Wednesday, and in honor of its anniversary, Apatow took to Twitter to answer fan questions about the making of the film.

One fan asked Apatow whether an actual, real-life moment had inspired the film, and it turns out, the answer is yes.

“I was talking to Seth and said ‘I think you don’t need to be in a movie with special effects. You would be funny in everyday situations. You could just accidentally get a woman pregnant and it would be funny,'” Apatow replied.

And just like that, the plot of Knocked Up was born — no pun intended. Rogen and Katherine Heigl star as a slacker and a budding TV journalist who realize they’re going to be parents after a one-night stand gone wrong. Leslie Mann, who is married to Judd Apatow in real life, and Paul Rudd star alongside them as a bickering married couple.

The cast of Knocked Up also features B.J. Novak, Kristen Wiig, Jay Baruchel, Jonah Hill, Jason Segel, Martin Starr, Ken Jeong, Bill Hader, Adam Scott, Stormy Daniels, Craig Robinson, and Apatow and Mann’s children Maude and Iris Apatow.

Another fan asked how Judd Apatow kept chemistry between actors flowing on set, and whether this was the first movie roles Maude (Euphoria) and Iris (Love) ever got.

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“Chemistry was easy because everyone was really good friends and there was tons of natural dynamics to tap into,” Apatow replied. “Maude and Iris had been cut out of other movies before this! Maude had a funny run in Talladega Nights cut as a kid who knew tons about racing and talked fast.”

One Twitter user asked Apatow whether he would change any scenes if he could, to which he brought up the late, great Harold ramis (Ghost Busters, Groundhog Day).

“I wish we did more scenes with @Sethrogen and Harold Ramis. They were so great together,” Apatow replied. “Tons of improv that day at The Pantry in downtown LA. A perfect, fun afternoon with Harold – who we miss dearly. We knew if he was Ben’s dad the audience would know Ben was a good guy.”

Another fan asked how Apatow ended up casting Rogen and Heigl as the leads.

“We read everyone in town and so many were fantastic actresses but the moment Katherine read everything they both did became really funny and right,” Apatow said.

Apatow also responded to a tweet about the dinner scene in Knocked Up in which Rudd and Rogen’s characters bond over their shared dream of using the Delorean time machine from Back to the Future to experience life before fatherhood, as Leslie Mann and Katherine Heigl’s characters roll their eyes.

“When I think of the Knocked Up shoot I think of the cast having to ride rollercoasters all morning and throwing up. Looking back, I did too many takes,” Apatow tweeted in response to the hilarious clip, which you can watch below.

Read more of Apatow’s tweets answering fan questions below.

Main Image: Seth Rogen and Katherine Heigl in Knocked Up. Credit: Universal Pictures