"[Courteney] knows she's not f***ing me, and she's like, 'Listen, I want you to be able to do whatever you have to do,'" he said. Arquette admitted to sleeping with Waltz, whom he called "the girl in the paper," "once ... maybe twice."
He also revealed how he broke the news of the breakup to their 6-year-old daughter, Coco.
"We went to a beach and had a picnic," Arquette said. "[I] explained to her that we're grown-ups, and a lot of the time when grown-ups are growing up, they need to figure out their lives."
At the time Arquette said he hoped the separation won't end in divorce.
"We're still working it out," he told Stern. "We're being very sweet and kind to each other."
But with both parties free to move on -- Arquette told Stern that he and Cox agreed they could be intimate with other people during their separation -- reconciliation may be less than likely.
"You've got two very different kinds of people who have probably not seen eye to eye for years. After the infatuation wore off, they probably realized that," said Helen Fisher, chief scientific adviser to Chemistry.com and author of the relationship analysis tome "Why Him, Why Her?"
"She's what I call the director, he's more of a negotiator," Fisher said. "I'm not surprised that she's being practical about this, and he's being syrupy, because the director type tends to be decisive and tough-minded. She probably found that he was sleeping with someone else and she's not willing to negotiate that. She would probably rather find someone else."
David Arquette was born on a spiritual commune in Winchester, Va. His brother, Robert, became a transwoman and actress, Alexis, in the 1980s. His other siblings -- Patricia, Rosanna and Richmond -- are also involved in show business.
Arquette himself has long marched to the beat of his own drummer. In 2000, he literally leaped into the ring after filming the movie "Ready to Rumble," produced by World Championship Wrestling. Later that year, he became the WCW World Heavyweight Champion and inspired a parody song about his dual acting and wrestling career, "You Cannot Kill David Arquette."