Longtime friends, Will Arnett and Bradley Cooper co-wrote and co-star in Friday’s “Is This Thing On?” a divorce reconciliation comedy-drama that Cooper also directed.

Arnett and Laura Dern are Alex and Tess Novak, who’ve divorced but continue to see each other to raise their two sons and mingle in the same social circle.

On impulse, Alex, who has moved into the city and a bachelor apartment, tries standup at an open mic night. It changes his life, including reconfiguring his status with Tess. Can they find love the second time around?

“Bradley and I have been friends for 25 years,” Arnett, 55, said in a virtual press conference. “I’d come across this story inspired by the real British comedian John Bishop who told me how he came to standup.

“I really connected with the idea of this guy who finds something that allows him to start speaking his truth at a time when his life is cratering. And what it does for him — and his connection to his wife. How, with their relationship, standup allows him to reimagine who they are later in life.

“When I started writing it, Bradley and I were traveling together. Because we’re so close, if I’m being honest, I just thought I could get some free notes out of Bradley when I’m writing this script with Mark Chappell.

“I sent the basic outline to him. He called a week later and said, ‘I want to direct this. If that’s okay.’

“So that’s how it came to be.”

To co-write, produce and star — did that affect Arnett’s performance?

“It wasn’t by design,” Arnett explained, “I came across this idea and started writing it.  Always with the hope that maybe I could play Alex Novak.

“Once Bradley came on, he solidified that: ‘Yes, I want to make it and I want you to be Alex Novak.  And produce it with me.’  All those hats — I didn’t choose them.  They just happened to be thrown on my head.

“I never really felt the weight of it in that way.  I felt very lucky to build the story, build Alex. I got to understand, hopefully, who Alex was and what he’s going through.”

It helped that Cooper, who also costars, was open to innovation, improvisation.

“A great example,” Arnett said, “was while we were shooting scenes in Oyster Bay. The night before, Bradley said, ‘I’m going to ask Andra (Day, who plays Cooper’s wife) to sing ‘Amazing Grace.’ Tomorrow.’

“So, he called the night before, ‘Would you be willing to do it?’  She was immediately, like, ‘Sure.’

“That kind of openness to roll with it,” was how it was. Making sure “Is This Thing On?” was really … on.

Will Arnett in a scene from “Is This Thing On?” (Photo by Searchlight Pictures/Jason McDonald, Courtesy of Searchlight Pictures.)