(CNN) -- After "will-they-or-won't-they" comes, "what now?"
Over five seasons, viewers of ABC's "Castle" have gotten to know Richard Castle, a mystery writer paired up with Detective Kate Beckett, and they've solved one murder after another, all the while fans were hoping these two would finally get together. At the end of last season, they got their answer.
After TV Guide readers voted them the favorite TV couple (not to mention the show placing in the top five favorites for CNN.com readers), it's clear that audiences are invested in just where the writers plan to take Castle and Beckett.
CNN.com spoke with "Castle's" creator Andrew W. Marlowe to find out why this couple and this lighthearted show have struck such a chord in a dark TV landscape. Plus, we found out what's to come leading into this spring's 100th episode.
CNN: How has this show evolved over five seasons?
Marlowe: Any show evolves. Hopefully we've retained our core values. We've had two characters who were forced together and there was friction, and sparks flew.
Of course, over time they got to know each other and a different story emerged -- this love story that's tentative and goes in fits and starts. It's been fun to mine that, but we've maintained the basic core of the show - delivering a great relationship story through the device of the procedural with fun, interesting cases at the heart of it.
But over five years, your characters do have to grow and evolve, otherwise it doesn't stay fresh and it's not honest, it's not true.
Since this was always supposed to be a "will-they-won't-they" romantic arc, the characters have to grow together, and go through the natural process of a courtship that everybody does.
CNN: To what extent did you map out the course of the show's romance?
Marlowe: Initially, you're always hopeful just to stay on the air, and once you're confident you'll stay on the air, you start to chart it out.
It was always our intention to get these characters together eventually, but part of the fun is how can you keep them apart naturally?
I didn't have a firm date [for the relationship to start] when we launched into it. You kind of feel your way through and see what the rhythm is and how the show is evolving. At a certain point I believe you start testing your viewers' patience when they have a vested interest in it. I've never believed in the "curse of 'Moonlighting'" or all those concerns people had about getting them together.
CNN: Why do you think audiences love this couple and this show?
Marlowe: Anybody who's been in a relationship understands the give and take. There's always some fun friction there. You're also attracted to the things you aren't, so the things that you love are sometimes the things that drive you crazy.
In a universe of procedurals that are pretty dark, the fact that you can deal with this form in a more lighthearted way is a relief for people. After a hard day's work, you turn on "Special Victims Unit," you're in the darkest place possible with people trying to solve this crime. I was getting to the point when I was conceiving this, thinking I don't want to bring that into the universe. I wanted to bring in something joyous even though we're dealing with murder.
I go back to a show like "M*A*S*H" where it's life or death stakes in the operating room, but there's also that gallows humor and that humanity. Hopefully we're finding that tonal balance in "Castle."
CNN: Has Nathan Fillion and Stana Katic's chemistry surprised you over the years?
Marlowe: Nathan was generous enough to sit with us as we read 125 or 130 actresses. There was something special in the connection that he and Stana had. There was a tension that was fun and interesting -- it was that lightning in a bottle chemistry that you look for, and we were fortunate to get it. We were thrilled with what we had.
CNN: What made Nathan right for the role?
Marlowe: The reason I thought he would be good here is this would be a character that would allow him to play everything he's good at. This would allow him to be dramatic; this would allow him to be funny; this would allow him to be charming. The role is written to show the different aspects of what it means to be a man: to be a lover, a long suffering son, a tremendous father, a man's man, to hang out with the boys. The role is written to see all those facets. If we only saw him annoying Beckett, or the narcissism he has, he would come across as very unattractive. The nice thing about the show is we get to see all the facets of the diamond.
CNN: What do you have coming up?
Marlowe: We're currently working on the 100th episode, but we have some big storytelling before then. Traditionally we have a two-parter this time of year. Castle's daughter ends up being kidnapped and put into a very difficult situation. We play a lot of it from Castle's point of view, trying to get her back. It's a tremendous performance by Nathan.
After the two-parter, we're doing a little nod to the horror genre, centered around a haunted videotape. Castle sees it and is convinced he's going to be dead in 48 hours. It's a ride; it's a lot of fun.
We find out Ryan has a past going undercover in narcotics, and he ends up having to go undercover again. It turns out he's a very different person undercover.
Later on this season, we have a murder where it looks like the only credible suspect is Bigfoot, which sends us on a Bigfoot hunt. We like to lean into the genre and end up with a real world explanation.
For the 100th we wanted the quintessential 'Castle' show -- something that was fun, something with great Castle-Beckett stuff in it. We're really excited with the direction it's going in.