Dermot Mulroney on death of ‘Family Stone’ star Diane Keaton

Dermot Mulroney is remembering the late Diane Keaton, who played his onscreen mom in 2005’s “The Family Stone.”

Keaton died in Los Angeles on Oct. 11 at the age of 79. Her cause of death was later revealed to be pneumonia.

“It broke us all up,” Mulroney, 62, exclusively told The Post while promoting his new western “Long Shadows.”

“I had lunch with Tom Bezucha, the director of ‘The Family Stone,’ the week before. So we spoke so much about Diane,” he recalled. “Even then, I didn’t know that she was going to go so soon. She’d just been on my lips a few days before. So it really impacted me. And I was in touch with him and a few other people around that film.”

Released in 2005, “The Family Stone” featured an ensemble cast made up of Keaton (Sybil Stone), Mulroney (Everett Stone), Craig T. Nelson (Kelly Stone), Luke Wilson (Ben Stone), Elizabeth Reaser (Susannah Stone), Claire Danes (Julie Morton), Tyrone Giordano (Thad Stone), Rachel McAdams (Amy Stone), Brian White (Patrick Thomas) and Paul Schneider (Brad Stevenson).

The Christmas comedy-drama begins with Everett bringing his uptight girlfriend, Meredith Morton (Sarah Jessica Parker), home to spend the holiday with his family.

Several disastrous incidents ensue, and the movie culminates with Keaton’s matriarchal character revealing that her battle with breast cancer has returned.

One of the film’s most emotional scenes centers on Keaton’s Sibyl telling Mulroney’s Everett that she is sick again. The Academy of Motion Picture Arts and Sciences chose to use that moment in their Instagram post to pay tribute to the “Annie Hall” star following her death last month.

“The scene that day, of Everett Stone and her in that scene where I learned that she’s ill again, that actually was really devastating to see,” the “Anyone But You” star told The Post. “Can you imagine the combination of having worked that closely with her to perform that scene as best as I ever could?”

Keaton was initially “pretty serious” on the set of “The Family Stone” before the cast managed to loosen her up behind-the-scenes.

“Diane went through a warming-up process,” he said. “People aren’t sure how strict they’re going to be or what the vibe is.”

“About halfway through the movie, of course, she was goofing around,” Mulroney continued. “We brought her down to our level. So by the time we got to the epic, iconic foot in the mouth of the century scene, at the dining room table, we were all just in awe of each other.”

While Mulroney praised all of his “The Family Stone” co-stars, he acknowledged that Keaton was the glue that held the cast together and really made the movie work.

“We knew that it was such a great group, perfect for these roles,” he explained. “By then, she was the leader that we embraced, rather than wondering how it was going to go.”

“She could have fed us crumbs, and yet she gave it her all, and we all got really close,” Mulroney added of Keaton’s celebrated performance. “So it was quite an incredible process. The filming of ‘The Family Stone’ had its own lifespan in that way. So many things happened during it.”

Despite the film’s heartbreaking plot, “The Family Stone” has become a classic Christmas movie for audiences.

For Mulroney, the movie’s lasting resonance 20 years later was not originally expected.

“It came as a surprise that it became a perennial, annual must-watch,” he explained. “Two Christmases ago, the [Wall Street Journal] did a lengthy article about why people were drawn to such a wrenching movie, and watched it every year to make themselves cry.

“So that’s to say it’s such a powerful movie and means so much to people and families,” he added.

As for whether there have been any talks about a possible “Family Stone” sequel, Mulroney revealed that there have been – although he remained tight-lipped about any additional details.

“Is there any discussion about making a follow-up? Yes,” he concluded. “That’ll be the answer that I leave as brief.”