The biggest challenge for Kristin Davis on the set of NBC’s Atomic Train wasn’t jumping over fires or running around in the rain for hours. It was playing the mother figure to costars Sean Smith and Mena Suvari. "I was locked in a Jeep with them for days, so it didn’t make me think, ‘Gosh, I want to have teenagers,�" jokes the actress, 34, who stars opposite Rob Lowe in the miniseries (May 16�17, 9 P.M./ET) about a train carrying nuclear weapons and waste that careens out of control.

Still, Davis, who is single in real life, did enjoy a role very different from Charlotte, her prudish art dealer on HBO’s Sex and the City. "It was a nice departure to actually play a woman who is responsible for other people than just for herself," the Melrose Place alum says. Now back in New York City shooting the second season of Sex (premiering June 6), Davis reveals Charlotte will be "even more picky about who she is going to go out with," and Sex’s four single women will be debating matters of the heart (and body) more vociferously.