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– Rosie O’Donnell has been a talk-show host, starred in movies and made guest appearances on TV series. But she hadn’t been a regular cast member of a series until the new Showtime dramedy “SMILF,” by Boston filmmaker Frankie Shaw.

“My agent sent me the short films that Frankie did, and I was blown away by both of them,” O’Donnell said. “My agent said very meekly on the phone, ‘This is not really an offer. You have to talk to her on FaceTime.’ I was like, ‘Get that woman on FaceTime. Let’s see what we can do.’

“And I was just absolutely shocked by her talent and the message and the kind of feminist perspective that she had in a very universal way. And I was really thrilled to be a 55-year-old woman and see a 30-year-old woman being able to make those two pieces of brilliant art. It was like a dream come true. I said, ‘I’ll do anything that she needs or wants if she’ll have me.’ And she said yes.”

“SMILF,” which premiered last weekend, is based on a Shaw short that won a jury prize at the 2015 Sundance Film Festival. Shaw plays Bridgette Bird, a determined and independent single mom trying to deal with her extremely unconventional family. Her struggles to make ends meet often result in immature decisions, but ones that are always based on making a better life for her son.

O’Donnell portrays Bridgette’s mother, Tutu, a tough, opinionated and narcissistic woman who’s resistant to change of any kind. Tutu struggles with manic depression and has a volatile relationship with almost everyone in her life, especially her daughter.

Because the series is loosely based on Shaw’s life, O’Donnell wanted to make sure she was doing justice to the character. She got the information she needed after splitting a couple of bottles of wine with Shaw’s mother.

“We were lucky, we got to hang out a lot,” O’Donnell said. “I spent Thanksgiving with Frankie and her family.

“I don’t have a mom. My mom died when I was 10, so to see what would have been my mother’s life in Frankie’s mother was kind of a beautiful and healing thing for me. You know, it’s working-class Irish people, and that’s who I was growing up, and that’s who you remain. It felt familiar, loving and really authentic.

“I liked her very much. I saw her vulnerability. I saw her self-doubt, and I also saw her pride in her daughter, whether or not she’s able to express it. And that will be an interesting challenge as an actress, to play a mother who sometimes chooses to disassociate what she’s afraid of — the intimacy.”

Shaw, who serves as executive producer of the series, said she knew in their initial chat that O’Donnell was the right person because she said she wanted to disappear into the role.

The role comes with serious demands, as Tutu faces the complexities of mental illness and occasional bouts of uncontrolled rage. While that might scare some actors, the opportunity excites O’Donnell.

“For me, it’s thrilling to be able to play somebody with mental illness,” she said. “I suffer from major depressive disorder and PTSD, and I’ve been medicated since April 1999, right after [the] Columbine [school shootings]. World events seem to be big triggers for me, so when I was talking to Frankie on that first FaceTime call, I said that was an issue that I really would love to explore in an accurate and realistic way.”

SMILF

When: 9 p.m. Sundays.

Where: Showtime.