We knew it was only a matter of time: In this fall's eagerly anticipated musical episode of Buffy the Vampire Slayer, the title butt-kicker's blood-sucking suitor finally rocks her world — literally. But James Marsters warns TV Guide Online not to expect a Celine Dion power ballad from his counterpart, Spike, Sunnydale's fanged Sid Vicious.

"We were in London on the dance floor, and Joss Whedon [the series's tuneful creator] actually stopped dancing, going, 'I'm in the middle of your song — I'm on fire, I can't get it out!'" relates the actor, who covers Tom Waits and Bruce Springsteen with his own band. "Just today he was like, 'The last stanza needs more balls.' He wants it to be really rock and roll."

However the number turns out, Marsters teases that a more harmonious relationship may develop this season, at least temporarily, between the resurrected Slayer and her undead paramour. "The fact that Spike is truly in love can motivate him to great acts of heroism as he tries to become the kind of man that Buffy could love," he observes, "or, if spurned, it could drive to him to great acts of villainy."

In other words, Marsters cautions, hopeless romantics — his character included — shouldn't become too hopeful that the odd couple can avoid hitting a sour note indefinitely. "I don't know if Buffy will ever reciprocate Spike's feelings," he admits. "I really think Spike is kind of beneath her. He's evil — he really is. He just happens to be in love with a good person."