Chemistry is one of the key elements of any relationship - and the same goes for the cast of a TV show. Friends had it by the bucketload, which helped to make it a huge success for all involved. Before that, the big hit of the 1980s was Moonlighting, the quirky private eye series that made a star out of Bruce Willis, largely because of his remarkable on-screen rapport with his co-star, the already established Cybill Shepherd.

Although it's nothing like Moonlighting, Castle has some of that show's properties. Firstly, the main characters have a sparky relationship that may or may not lead to romance. Secondly, it's great fun and could be a real hit with viewers who fell for The Mentalist, which as a similar, old-school, crime-solving remit.

The basic premise is that Richard Castle, a successful crime-writer, is approached by the NYPD when a series of murders appear to be copying plots from his books. He's questioned by Detective Kate Beckett, who instantly dislikes him, but the pair end up working together, in the way Patrick Jayne's works with Teresa Lisbon in The Mentalist. The one big difference is that there's a spark between Castle and Beckett that may or may not lead to romance. Got it? Good!

Castle stars Nathan Fillion - after Buffy, he was the lead in the superb but short-lived Firefly, before reappearing as Katherine Mayfair's husband Adam in Desperate Housewives. His co-star is Stana Katic, who was Corinne Veneau in the James Bond movie Quantum of Solace, as Beckett. Castle boasts a very strong supporting cast, includes veteran actress Susan Sullivan, who's got Dharma & Greg, Falcon Crest and Rich Man, Poor Man on her lengthy CV.

The RTÉ Guide is in a room at the rather swanky Renaissance Hollywood Hotel, just around the corner from Hollywood Boulevard, and next door to the Kodak Theatre, where the Oscars are held. The Hollywood sign is visible from the hotel's entrance, so you really can't get more Tinseltown than this place.

Relaxed and chirpy, both Nathan Fillion and Stana Katic are basking in the knowledge that the show is going well. Fillion, for sure, has had more than his fair share of ups and downs in this fickle industry. "Having been on a number of different shows so far . . . you can kind of gauge the temperature of our show", he reckons.

"It's so easy-peasy on set", says Katic.

"Right", adds Fillion. "Very easy. They've left us alone to do our own stories. It's been fantastic. And the support they've shown us with the promotion alone has been fantastic. I've never experienced the like. So they've shown us a tremendous amount of support."

It can't have done the highly regarded Fillion any harm appearing, however briefly, in a show as big as Desperate Housewives, although he plays down his time on Wisteria Lane.

"You know, I didn't have much to do on Desperate Housewives", insists the 38-year-old Canadian. "I sat around in the living room a lot and said, 'Yes? Uh-huh. Well, if you're going to lie, I'll back you up'. But I worked, like, maybe two days a week. The pressure was very low on that show. All the women do all the heavy lifting. But I will also say that I think probably more people saw me on Desperate Housewives than in all the rest of my stuff combined."

The one thing that could propel Castle into the big time is the chemistry between Fillion and Katic. "He was cast first", says Katic. "And you're sitting in there, you're reading this character who's supposed to be attractive, and he's talented. So it was easy. It was fun. It was very exciting. I'm very lucky to have Nathan as a partner to play with on this show."

Fillion, meanwhile, sees the job as a "piece of cake" due to Katic's acting chops. "I'll tell you this: Stana can play chemistry with a wet paper bag. So, again, no heavy lifting. You don't have to do much. She has an intrinsic understanding of how to play it and just in how to find the language and do the stuff. We have a lot of fun."

Katic is probably best known for playing Canadian spy Corinne Veneau in Quantum of Solace. Playing opposite Daniel Craig certainly went well, judging by her smile when the topic's brought up. "You know what's funny, though?" she offers. "Daniel has a wonderful, beautiful sense of humour. We don't get to see that necessarily through his James Bond character. But he has a fantastic boyish sense of charm inside as well. It was really nice to work with him. As does Nathan."

"Yes", adds Fillion in a flash, smiling at Katic. "I have a very Daniel Craig sense about me."

As for the Moonlighting, 'will-they-won't-they' comparisons, Katic freely admits that the Shepherd-Willis show is influential on Castle and its main characters. "I think we're kind of referencing Moonlighting more", she feels, while drawing in other influences from Hollywood screwball comedies. "There's kind of a tipping of the hat to classic shows and movies like His Girl Friday and The Thin Man and so on. So I think we're not going to give it to you all at once. There's a lot to be discovered and unearthed in each character before they they're attracted to each other . . . We're letting the audience get to know each character first. And there's a lot to learn about each of us."

Fillion is keen to fill in some of the blanks for prospective viewers, from his character's point of view. "I'll tell you a little bit about what Castle's learned about Kate Beckett. Not only is she beautiful; she's intelligent and she's very much a challenge. I think things come very easily to Richard Castle, but certainly not Kate Beckett. She will not be fooled. She will not be had. She will not be taken in or charmed. She will not have it. And she's very, very driven. She's experienced a personal tragedy that instils within her a desire for justice. She needs justice more than anything."

Katic returns the favour, giving us Beckett's view of Richard Castle. "What my character has seen of Castle is this initial cad and this initial playboy. And she discovers the other side of that character as we move on in the series. He has a family, he has a daughter, he has this wonderful Broadway mother. She sees the heart in Castle eventually, which, I think, is what engages her. He's a fantastic crime-solving team-mate."

Fillion points out that there are reasons why his character has a bit of growing up to do. "Because Castle is a man surrounded by women, he lacks any kind of male adult role model in his life. And I think that's part of the reason he stayed so boyish and playful and kind of Peter Pan-ish. He doesn't really grow up. He's still into toys. He's still into fun. He's having a great time. He's kind of mothered by his 15-year-old daughter while he sort of mothers his own mother. It's a very unusual nuclear family."

Most of all, though, watching Castle is one of the better ways to spend a TV hour. Enjoy!

The Castle crew

Here's a handy, getting-to-know-you list of the key characters in Castle.

Nathan Fillion is Richard Castle
A well-known author of crime novels, he learns that a copy-cat killer is staging murders depicted in his novels. He gets questioned by NYPD Detective Kate Beckett about the real murders and sees it as an opportunity to use his knowledge and skills to catch criminals.

Stana Katic is Detective Kate Beckett
A bright and aggressive homicide detective with the NYPD, Beckett is a first-class investigator who has gained a reputation as a detective who's intrigued by unusual cases. She joined the force after her mother was murdered and the case was never solved.

Jon Huertas is Detective Javier Esposito
Works in the homicide division with Kevin Ryan as part of Beckett's team. He is always ready with a sarcastic comment to keep Beckett on her toes and enjoys the way Castle riles her up.

Seamus Dever is Detective Kevin Ryan
Works alongside Esposito. Both of them regularly tease Beckett, but they are also incredibly protective of her and ultimately treat her like one of the guys.

Tamala Jones is Dr Lanie Parish
A medical examiner and a friend of Beckett's, she is one of the few people Beckett can talk to easily. She is also one of the few people to notice the chemistry between Beckett and Castle.

Ruben Santiago-Hudson is Roy Montgomery
Beckett's boss who has a father-like attachment to her. He is amused by how much Castle annoys Beckett, but also appreciates how well they work together as a team.

Molly Quinn is Alexis Castle
Castle's 15-year-old daughter is more mature and responsible than her father - which says a lot more about him than her. She is very bright and enjoys school.

Susan Sullivan is Martha Rodgers
Castle's Broadway diva mother lives with him and Alexis. She helps her son through the difficul