Molly Shannon is looking back on an unforgettable moment she had with late pop icon Whitney Houston.

The comedian, 57, is making her rounds promoting her new book, “Hello Molly!: A Memoir,” and stopped by Howard Stern’s eponymous radio show Tuesday.

Shannon collaborated with Houston — who died in 2012 at age 48 — on a “Saturday Night Live” sketch in the ’90s involving the former’s infamous Catholic schoolgirl character, Mary Katherine Gallagher.

The “Wet Hot American Summer” actress described to the shock jock, 68, how the sketch involving Houston came to be.

“Whitney was so nice. They’re like, ‘She’s not going to be in the sketch. She’s not going to do it,’ ” Shannon said. “I was like, ‘She’ll do it. Let me go talk to her.’ “

The “I Will Always Love You” songstress was the musical guest on the Dec. 14, 1996 episode of “SNL.” Rosie O’Donnell was the host of that evening’s show and Penny Marshall made an appearance.

The sketch featured two nuns (O’Donnell and Marshall) prepping their students for a choir competition. Houston played “the perfect Jennifer,” a snobby popular girl who Mary Katherine tries to “out-sing” with the holiday tunes “Little Drummer Boy” and “Sleigh Ride.”

Shannon went on to note that while the Grammy winner was happy to do the skit, the “White Lotus” actress was worried that the singer would not show up.

“Sometimes I would see where they would approach the musical guest and they would have too many lines or it’s too confusing,” Shannon added. “And I thought, ‘They don’t want to have all of this pressure memorizing these lines.’ So I just told Whitney, ‘Look, I’m going to play the Catholic school girl. You’re going to be like a snotty girl. All you have to do is, just do whatever you want. You just have to be snotty, push in front me, out-sing me. You can say whatever you want.’ “

Shannon continued, “She was like, ‘OK, I’ll do it, I’ll do it!’ So that’s how I got her to do it. And that’s what I meant. I was like, ‘Don’t even look at these papers. Just have fun!’ “

When it was time for the show to air, “SNL” alum Ana Gasteyer was all dressed and ready to take the “Bodyguard” star’s place, just in case.

“Then the night that the sketch was on, she was not there yet. And I was like, ‘Oh no.’ So they had Ana Gasteyer dress in the Catholic school outfit, so she was ready to go onstage,” the Ohio native said.

Happily, Houston kept her promise.

“It was like, honestly, like five seconds, they’re counting down, [Gasteyer]’s about to go on,” Shannon said. “And then they’re like, ‘Whitney’s here, Whitney’s here!’ They usher her on, and she was fantastic. She showed up.”

Shannon also explained how Houston and former talk show host O’Donnell interacted. “Rosie was great. She was so good, and Whitney was so nice after,” Shannon stated. “She loved being in that, and she was like, ‘Girl, you’re crazy.’ She had so much fun. She had such a good sense of humor.”