Liev Schreiber is doing damage to a platter of salami and eggs at the City Bakery in Brentwood, trying not to think about cigarettes. After smoking more than a pack a day for 23 years, he has just given them up. There's no sign of nic-fitting in his equable gaze, but he promises his nerve endings are in knots. "I'm on Chantix. I chew gum. I went to a hypnotist. I had acupuncture yesterday. I'm trying everything you can possibly do to quit cigarettes. But I think the way I'm really doing it is willpower," he says. "It's the baby."

Back home at his nearby mansion are the loves of Liev's life, the incandescent Naomi Watts and said newborn, Alexander Pete Schreiber, born July 25 weighing eight pounds four ounces. The giddy new father checks his iPhone constantly and worries about, among other things, whether Watts will disapprove of his robust morning meal.

Schreiber, 39, has busied himself lately with constructing baby furniture and loading batteries into electric toys, but he recently snuck away to Cartagena, Colombia, for a small role in Love in the Time of Cholera, based on the novel by Gabriel García Márquez. In perhaps not his greatest acting challenge, Schreiber plays the able lover Lotario Thurgot, lusted after by women everywhere. "I did it as a favor to a friend," he says—and because he'd never been to that part of South America. "I was convinced there would be wild orgies happening everywhere," he says. "There weren't."

Not that Schreiber's really the orgy type. "I can't imagine going anywhere except the La-Z-Boy and just looking at him," Schreiber says, still adjusting to Pete's presence in the world, "holding him on my chest, seeing his face." He's anxious about the new responsibilities of fatherhood but eager to think about someone other than himself. Watts, also 39, has read all the parenting books. Schreiber's going with more of an intuitive approach. "It's not so much my strength I have faith in," he says. "I have an infinite amount of confidence in the strength of children."

Schreiber, a quicksilver actor who shape-shifts with ease from Orson Welles to that creepy guy in the Scream trilogy, starred opposite Watts in 2006's The Painted Veil and wrote and directed the adaptation of Jonathan Safran Foer's Everything Is Illuminated—despite his broad contempt for the filmmaking process. "Movies are pretty unpleasant," he says. "You shuffle back and forth between being skinned alive and being the most fulfilled and happy you've ever been in your life." His most underrated work has been on stage, where he has played Hamlet, Macbeth, Henry V, and the expletive-spewing philosopher–real estate agent Richard Roma in David Mamet's Glengarry Glen Ross, which earned him a Tony in 2005. While he carefully weighs his next stage appearance, he's filming Defiance with Ed Zwick.