With makeup, lipstick and a good blow-dry, Elizabeth Mitchell can get through a morning of errands incognito. But in sweatpants, a ponytail and a touch of busy-mom-au-naturel grunge? That's when the neighbors figure out that Juliet—Lost's gun-toting, Jack-smooching, loyalty-shifting femme fatale—is among them. Says Mitchell with a laugh: "I get recognized instantly."

Not that Mitchell, 37, minds showing a little sweat. Her Lost role—as a doctor who left the malevolent "Others" to join the castaways—is the juiciest yet for the journeywoman actress, who made her mark in movies and shows including Gia, Frequency and ER (see box). Is Juliet a double (or even triple) agent? "Part of what makes her so interesting is she is so maternal but she's also snakelike," says Mitchell. And in the new season—eight episodes were shot before the writers' strike halted production in November—"you can't really count on knowing anything."

While she misses her castmates (who keep in touch by e-mail), the strike has meant time to nest with her husband of three years, actor Chris Soldevilla, 38, and their 2-year-old son C.J. in their 1896 Victorian home on a small island in Puget Sound, near Seattle. Though it's a lot chillier than Lost's Oahu set, she can hike in the woods and spend afternoons reading to C.J. by the fireplace. Motherhood is "insanely difficult," she says. "But truly, that whole cliché 'It's the hardest job you'll ever love'? It's true. He's a little curmudgeon—and I was too."

A self-described dork who "had super-frizzy hair and terrible acne" growing up in Dallas, Mitchell was encouraged by her lawyer parents to pursue theater. In 1994 she landed a short-lived role on the soap opera Loving. "I got fired. The fans hated me. The girl that played the character before was buxom and sexy, and I was very angular and odd," Mitchell says. "It was like replacing Pamela Anderson with Emma Thompson."

She rebounded with steady work on TV shows such as L.A. Firefighters and JAG and broke through to movies as Tim Allen's wife in 2002's The Santa Clause 2. That year, she had her first date with Soldevilla (an acting teacher known for starring in ads for chain Del Taco). "He drove us to Topanga Canyon beach with two bottles of wine and a chess set. Then we went for a walk as the sun was setting. We jumped in the water. It was cold!" They wed in 2004. In 2006, just a month after they decided to leave L.A. and set up house in the tranquil Pacific Northwest, she landed Lost.

The whirlwind is worth it, says Mitchell, but while she's waiting to go back to work, she's content to take lead vocals when she plays Guitar Hero with C.J. on drums and Soldevilla on guitar. "This just suits us more," Mitchell says of life in the slow lane. "It's a really good life."