Paula Pell stops short, grabs my shoulder and yells, completely serious, “Oh! My journal! Oh! Jesus Christ!” We are about seven steps from the booth we just ate in. “Oh, I have it,” she says, sighing with relief after finding the yellow spiral notebook secure in its leather cover. She wrote this diary when she was 13, long before she started reading it aloud late at night to entertain the staff of Saturday Night Live–and even longer before she turned it into the movie Sisters (Dec. 18), starring two women who over the past 14 years she has spent more time with than most real sisters do, Tina Fey and Amy Poehler.
“Tina heard that journal 50,000 times–she could quote from it,” says Pell, 52. So when she thought about using the diary for a one-woman show and her agent suggested something more ambitious, Fey immediately offered to produce a film. It then became one of the easiest casting jobs in history.
For Pell’s screenplay, the stars reversed the dynamic of their last movie together, 2008’s Baby Mama. This time Fey plays wild older sister Kate, and Poehler’s Maura (kind of sounds like “Paula”) is the one who lends her money. When the siblings find out that their parents are selling the Orlando house they grew up in, they fly back and throw one final rager.
The actors have been tight with Pell for so long–she started writing for SNL in 1995, Fey came on board two years later, and Poehler joined in 2001–that they already knew Pell’s sister Patti as well as her parents and nieces, one of whom did hair for the film. “There’s an old improv rule to already know each other when you start a scene,” Fey says. Poehler adds, “We didn’t have the usual dance you have to do at the beginning of any project.”
Instead of being offended by Fey’s portrayal of Kate, Patti was amused that an actor she’d known for so long was caricaturing her. “My sister laughed really hard when I showed her the money stuff,” Pell says. “I’d always be loaning my sister money, knowing full well I wasn’t going to get it back. But she had the kids, and that paid me back.” “Yeah,” Poehler adds, nodding sweetly. “It’s not the same as money.”
Sitting with Fey and Poehler, Pell is the loudest, funniest, least adult one. A product of suburban Orlando and a lesbian, she exudes sloppy bawdiness wrapped in tightly wound Southern maternal kindness. At SNL, she helped Will Ferrell and Cheri Oteri hone their Spartan-cheerleaders sketches, turned Debbie Downer into Rachel Dratch’s most memorable character and crafted an absurdly optimistic version of Tony Bennett for Alec Baldwin to parody. Fey eventually became Pell’s boss as head writer, and they became so close that Pell found her an apartment in the same Upper West Side building she lived in; when Fey moved out, Pell took her place. “We’ve known each other from the first day Tina came to SNL,” Pell says. “Tina and I became fast friends, and I met Amy through Tina. I’ve witnessed them both having amazing children, whom I love and are excruciatingly cute.”
In Sisters, Kate makes a fearful diary entry about getting pregnant in high school while Maura writes about how the new grit in her rock tumbler is really shining up an amethyst. This comes directly from the Pell siblings’ actual diaries. When I page through Paula’s yellow journal, I run across this: For a person with a lasting cold & a cough, I feel great. I sure hope this mood of mine lasts forever. I love you God.
Fey and Poehler also contributed slices of their teen years. Their characters’ childhood rooms, untouched since they moved out in the 1980s, are re-creations of the actors’ own rooms, decorated with posters of Michael J. Fox, Xanadu and Out of Africa. Poehler says her muscle memory came right back when she put on the Jane Fonda workout tapes.
During the shoot, Pell felt free to blurt notes to her friends, often handing one of them a new joke on a Post-it note to see how the other would react. One day, she quietly walked into the set’s fake bathroom, sat down on the toilet and started yelling at Fey. “She pulled down her pants and gave notes, pretending to go to the bathroom,” remembers Ike Barinholtz, who plays Maura’s love interest. “‘Tina, make sure to get a take … er! … where … uh! … it’s not too sentimental.'”
Used to ending SNL sketches with something big and silly that could be shot simply on a stage, Pell wrapped up the party scene by having them witness a giant sinkhole. What she didn’t realize about movies was that if Universal Studios was going to build a real sinkhole, the characters were going to have to spend some time inside it. “Tina spent a lot of time in that sinkhole,” Poehler recalls.
“Whatever the blue paint in the water was, it just melted a set of hair extensions–I had to throw them away,” says Fey. As they shot more of the scene, the actual house they built started to rot. “We shot all this stuff with James Brolin and Dianne Wiest as our parents, and we didn’t see them for weeks. Then they came back, and they were appalled,” says Poehler. “It’s a movie that smells as good as it looks.”
Sisters is just one part of a breakout year for Pell, who is developing a sitcom for HBO, has written another movie about sisters with Judd Apatow and is set to appear in two upcoming films. Mostly, though, she just wants to keep creating with people who are close to her, especially Fey and Poehler. “I’ve been on trips with them, had them slumber-party it during a blizzard at my house in the Hudson Valley. Tina and I took refuge with each other on 9/11,” she says. “When we shot the bathtub scene in Sisters, it was in the middle of the night. I walked in to them sitting in the tub, and I burst into tears. They are my family.” It takes a special kind of family to bathe together in their 40s.
This appears in the November 30, 2015 issue of TIME.