LOS ANGELES (Zap2it.com) - "This was an interesting week and a half, man," says James Marsters of his latest episode as vampire Spike on the WB's "Buffy, the Vampire Slayer."

"God .. I don't know how to say ... I think anybody who's had a metamorphosis in their life is going to be very interested in this arc, anybody who feels that they've transformed in their life, trying to become themselves."

As "Buffy" fans know, Spike, a longtime recurring character who became a regular this season, is a bleached-blond British bad boy (Marsters, by the way, is a California native), with a long black coat, short black fingernails and an excess of attitude. He has killed two Slayers and fancies himself a bit of a legend, even if all his attempts at world (or even Sunnydale, CA) domination fail miserably. But as with all people (vampires having once been people), Spike had a mother, a childhood, another name (of which we know the first part, William, as in William the Bloody) and another life before being turned into a demonic creature.

That life, Spike's origins and adventures, and the myth of the Slayer's invincibility lie at the heart of a Nov. 14 crossover event on the WB, beginning with an episode called "Fool for Love" on "Buffy" at 8 p.m. ET, and finishing with "Darla" on its spin-off series, "Angel" at 9 p.m. ET.

Earlier this season, Darla (Julie Benz), the 400-year-old vampire who turned an Irish wastrel into the vampire now known as Angel (David Boreanaz) returned from the depths of hell to haunt her now reformed protégé's dreams and life, in both flashbacks and in the present. Also seen in flashback has been Drusilla (Juliet Landau), a mad clairvoyant turned into a vampire by Angel, who eventually wound up siring Spike and being his paramour (before dumping him for a demon).

All these characters come together in the crossover, with both episodes featuring flashbacks to Victorian England, Europe and China, as the foursome of Angel, Darla, Spike and Drusilla wreak havoc across continents. This is the journey that eventually led them to Buffy's (Sarah Michelle Gellar) home of Sunnydale, and later led Angel to Los Angeles and his own series.

"Her story is epic," says Benz of the mercurial, sexually charged Darla. "To play a character like that on television, you don't get that opportunity very often. I mean, she lived four centuries. They really explain a lot about who she was before she was made a vampire. When they finally nail it down and give you what it is, it's really quite amazing."

"It's been great to go back and fill in the backstory," says Landau. "We were in London in 1860; we were in Yorkshire in 1880; we were in a Gypsy village in 1890; and then we were in China in 1900. In the China section, we were in the Boxer Rebellion, so there were a hundred extras and oxen, it was so lush."

In a recent episode of "Angel," Darla talked about how cruel and remorseless Angel was before a Gypsy curse restored his soul and conscience, and how such a capacity for darkness cannot be taught, but is innate. In the "Buffy" universe, transformation into a vampire doesn't necessarily change a personality, it just brings out hidden aspects.

Fans know that Spike fell hard for Drusilla, and lately his vehement and vocal hatred of Buffy is beginning to shift into something very different.

"That was the most interesting thing about the character from the very beginning," says Marsters. "Whereas he was truly evil, he was also very much in love with the girl."

In life, then, was Spike, or William, the punk-rocker hardcase he is now? As it turns out, nothing could be further from the truth.

"I think that opens doors for things that are much more interesting," says Marsters. "What was interesting was to try to find out what was exactly the same."

And what was that? "The passion, in the end" says Marsters.

"Just fire, fire that was misdirected for a long time, then got straightened out in a sick way by becoming a vampire. I'm still too close to it, it's hard to talk about it. But I started by dissing the man, 'I'm going to play sissy William now,' but I'm actually coming to love and respect him in my own way. The crew will be, 'Hey, there's poncy William,' and I'll be, 'Shut up!' " Marsters laughs at himself. "It's strange to talk about these things with a TV show."

One wonders what William would have been had he not become a vampire.

"Oh god, boy, there it is," says Marsters. "Yeah, what happens to a raisin in the sun? When you have great passion, if you don't find a way to channel that and turn it into something useful, it can eat you alive. For me, it was an episode of this guy discovering that he has a source of power, that he is a powerful person, but didn't know it yet. I think that's the metaphor that Joss (series creator Joss Whedon) was going for."

While both good and bad in the universes of "Buffy" and "Angel" may wear the coolest clothes and strike the sharpest poses, it's usually just a prelude to a pie in the face. These aren't Anne Rice's impossibly elegant bloodsuckers or a comic book's infallible heroes.

"No cool character really lasts very long on Buffy," says Marsters. "Joss is going for something much more human and much more real than something so overtly cool, which really doesn't exist in real life, it just exists on TV shows. What drives him is finding the foibles and the failings and the weaknesses and the things that make people so dear and wonderful and a little sad."

When asked last summer about whether he thought Spike's origins would ever be explored, Marsters said he hoped they would. But, having seen the flashbacks of Angel, in which Boreanaz sported a long wig and muttonchops, Marsters feared that a trip back in time might hold something similar in store for him.

"Ha!" he says. "I found out there's so much more to fear than muttonchops."