By Sara Vilkomerson
December 10, 2015 at 12:00 PM EST
  • Movie

One look at the video above, and it’s clear: Paula Pell, the screenwriter of Sisters — about two grown sisters throwing one last blowout house party — is the movie’s behind-the-scenes secret weapon.

“Paula is the funniest person in the room among the very funniest people,” says star Amy Poehler. “Everybody that works with Paula and gets to know her within five minutes knows that.”

One of Pell’s strengths, appreciated by her actors and displayed in this clip, is to come up with jokes as the scene is being shot, write them down on a Post-It note, and slide them to the actor.

“Usually someone yells out a joke to you form the director’s monitor where they’re watching, or someone else yells it out on a microphone and they get the laugh first,” says Poehler. “When Paula comes up to you and slyly shows you the joke, it’s a delight and a secret that you can’t wait to read.” (Adds Poehler’s costar and the film’s producer, Tina Fey: “Coming from an improv background, it is the most delicious form of cheating.”)

“I just love the Post-It system,” Pell, a longtime fixture in the writers’ room at Saturday Night Live, tells EW with a laugh. “I keep them all, too. I still have some from when I worked on This is 40 and Bridesmaids.”

In fact it was on Bridesmaids that Pell began the practice. “It was the first time I had ever been on a big movie set,” she remembers. “I didn’t feel appropriate walking on to the set [with a new joke]. So I just started scribbling line or idea alternatives and handing them to [director] Paul Feig.” She laughs: “I write them in a flurry.”

Sisters director Jason Moore guesses there were over 400 Pell Post-Its during filming. Take a look and see some of them in the video, Sisters: The Alt Vault, above.

2015 movie
  • Movie
  • R
  • 118 minutes
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