On most days, Robin Williams expends enough manic energy to power entire cities. But during a family trip to Paris on March 16, the usually kinetic comedian was anything but. Strolling through the Marche aux Puces flea market with wife Marsha and their kids Zelda, 18, and Cody, 16, he "seemed distracted," says an onlooker. And when photographers spotted them shopping two days later at an antiques mall, "he wasn't his usual self. Usually when he sees a camera, he's playful. This time, he was really quiet. Just glum."

The reasons for his un-Robin-like demeanor became surprisingly clear a few days later back home in San Francisco: On March 21 Marsha filed a divorce petition to end their 19-year marriage, citing irreconcilable differences. Friends of Robin, 56, and Marsha, 51, say that their relationship never recovered from the strain caused by Robin's alcohol relapse in 2006 after 20 years of sobriety. While Williams entered rehab and by all accounts has remained sober ever since, "the trust was broken," says a close friend of the couple. "She's hurt." They quietly separated on Dec. 31 (she stayed in their San Francisco home with Cody; he moved to Marin County), but "the marriage," says another pal, "has been over for a long time." Still, the split is "amicable," says Robin's longtime pal Richard Belzer. "It's not a vicious, bitter thing."

"It's sad," says Mike Medavoy, who produced Robin's 2007 comedy License to Wed. "She seemed solid and good for him." They had weathered tough times together right from the very beginning. When they first met, Robin was married to Valerie Velardi; Marsha worked for them as nanny to their son Zach, now 25, then later as Robin's assistant. After that marriage ended, Robin and Marsha fell in love in 1986. "She's a gentle, great soul," he told the Los Angeles Times in 1990. They married in 1989 and soon began collaborating professionally as well: Marsha produced his films Mrs. Doubtfire, Patch Adams and Jakob the Liar.

But their solid foundation began to crumble in 2006, as Robin entered rehab for alcoholism (he had also battled cocaine in the 1980s). Still, he put on a brave face, crediting Marsha last year for supporting him through his struggles. "She's amazing," he told PEOPLE. "She makes it look effortless."

Now that the effort has become too much to bear, both Robin and Marsha "are sad but going on with their lives," says a friend of the pair's. Marsha is focusing on her philanthropy work in San Francisco, while Robin is finding solace in acting, playing an anarchist in an episode of Law & Order: SVU slated to air April 29. "It's good for him," says a Williams friend. "He's always so energized by work." Agrees another pal: "Robin is a survivor. He'll be okay."