In the three months since True Blood first aired in the UK, it's been smashing viewing records in the States on a weekly basis, has turned its stars into tabloid staples and has given the vampire zeitgeist a much-needed grown-up twist. If you missed it on the FX channel and skipped the illegal downloads, here's the set-up: Six Feet Under creator Alan Ball is in charge and it's about vampires and sex, and sex with vampires, who now openly live among people and fight for their civil rights. Series one centres on Sookie Stackhouse (Anna Paquin), a telepathic waitress from Bon Temps, Louisiana, and her blossoming romance with Bill Compton (Stephen Moyer), who's a little old-fashioned about courting and affairs of the heart, since he became one of the pasty undead in 1868.

There's a lot of angst and sexual tension and will-they-won't-they and blah blah blah ... But the show has bigger fangs than the rest, and woven into its trashtastic supernatural storylines is a hefty dose of funny. That's mostly down to Sookie's job-incompatible, mouth-almighty best friend, Tara, and her drug-dealing, burger-cooking, road-paving, eyeshadow-wearing, redneck-bashing cousin Lafayette.

"People are crazy about it!" marvels Rutina Wesley, who gets the pleasure of being professionally shouty as Tara each week. "People are screaming and yelling at the TV! They love this show."

To Nelsan Ellis, who channels Lafayette with a deft swish and swagger, the popularity of his character was a complete surprise.

"I thought it would be the opposite," he says, in a slow, considered drawl that's much quieter and less cocksure than Lafayette's rapid-fire wit. "Lafayette is so risky, and he touches on a couple of different cultures. These are taboo things, so I didn't know if I would offend anybody or if people would be upset with me."

When he says "taboo things", he means that as a black, gay man, Lafayette is an oddity on TV. "In the black community, these things are frowned upon," he explains, carefully. "For it to be embraced by the black community, I was like, 'Oh, that's surprising.'" You could argue that it isn't so surprising because there's already a precedent. Look away now if you don't want to see the obligatory Guide reference to The Wire but Baltimore gangster Omar Little's sexuality is incidental to his hugely popular character, because he's written well and fleshed out. The traits Lafayette shares with Omar are the ones Nelsan thinks make him so appealing. "He's a hustler, a survivor - he's honest. So I think whatever isms someone may have, they sort of lose or forget about it."

Forgetting about it hasn't been quite so easy for Nelsan himself, who grew up in Alabama with strictly religious parents. "My mom's side, they're southern Baptist Christian. My father is Church of God in Christ, so you know what that means," he sighs. "It's hard dealing with them in terms of that. My mother hasn't seen a single episode. Neither has my father. They'd find it too difficult." Nelsan's scenes include screaming at an offensive redneck then throwing an "Aids burger" in his face, dancing in a gold thong in front of a webcam and introducing Sookie's brother to the dangerous drug V (vampire blood). You can see why his family might want to avert their eyes.

More puzzling is the fact that Nelsan - who's straight - is a devout Christian, even though True Blood regularly takes aim at the concept of organised religion in its portrayal of fundamentalist anti-vampire group The Fellowship Of The Sun. So how does he reconcile his faith with working on a show like this? "That's partly the problem my father has with me," he explains. "But I'm an actor ... I empty myself out and fill myself with the character. I would play a devil worshipper and I would fill myself up with whatever devil worshippers believe. Then, as myself, I empty that out and become Nelsan."

Rutina Wesley confounds expectations almost as much as Nelsan does. In the show she's a southern hothead with a troubled family life who can't hold a job down, mainly because she's thrillingly rude to any customers unlucky enough to cross her path. In real life, she's friendly, chatty and excitable. And two minutes into our phone conversation - she's in LA, where she lives now, though she's from Las Vegas - and we're talking about The Vertical Hour, the David Hare play she was in on Broadway before True Blood, directed by Sam Mendes and starring Julianne Moore. "There's nothing like the theatre," she says. "That's why I do what I do, to move an audience, and theatre is my favourite because it's live." Such luvviness might come as a double surprise to those who know her from her starring role in dance flick How She Move. But there's a reason she mixes it up so much. "It's partly because I'm an African-American actress," she says, "and me not wanting to get pigeonholed or stereotyped. So I made sure I trained in everything. It's not like, 'Oh you need dialect? Sorry, I didn't do that.' It's like, 'What accent do you need? Southern? Let me get my tapes.'"

Nevertheless, some critics in the States have accused the show of stereotyping its black characters. Newsweek's Joshua Alston wrote that he was giving up on True Blood because "Tara and Lafayette take turns filling the role of the sassy black person who says what the audience would be thinking if the audience's thoughts were quippier." The Guide puts that to Rutina, who explains the reaction to Tara in the States was initially far more negative, though people warmed to her as the first series went on. She's heard it all before and she's quick to defend the writers. "It's not like they all sit around a table and go, 'How do the black people talk? I think they talk like this.' They write characters and human behaviour. And Alan Ball really has a handle on human behaviour." Nelsan is similarly dismissive and puts it all down to a kneejerk reaction. "Black culture, when they see certain things like 'black woman loud', they immediately jump to the fact that it's one-dimensional, before they even watch the whole freakin' season," he says, before pointing out that Alan Ball is far too experienced to fall into that trap. "To accuse him of doing that, one has to suggest that you're absurd and you have no taste."

Why, then, aren't there as many "I ❤ Lafayette" or "Team Tara" T-shirts as there are Bill-themed garments? Maybe their romantic stories are too messy and their everyday problems too real and ordinary. True Blood needs that humanity to balance out all the saucy, gothic doomed love, but gritty realism isn't ever going to be the focus. The solution? Turn both of them. "Personally, I kinda wanted to be a vampire," chuckles Rutina."You get the teeth and to be cool and have cool clothes and be sexy all the time."

Season three has already been commissioned: she might want to start avoiding silver and sunlight.

• True Blood finale, 10pm, Fri, FX; the series starts from the beginning on 5 Oct, 10pm, C4

Nelsan on Lafayette

'He's risky, he touches on different cultures'

Rutina on Tara

'I wanted to be a vampire, you get teeth!'