Nathan Fillion

If you settle for 20th century definitions, Nathan Fillion is a prime-time television star – after all, his Monday night series “Castle” is cruising through its fourth sly season on ABC with more than 13 million viewers a week. But in this pop-culture era of digital tribes it’s really not fair to limit his celebrity with that sort of remote-control thinking.

These are the days of compartmentalized fame and there are few better examples than Fillion, who is able to able to anchor a popular network series even as he puts together a dynamic resume of cult-audience projects, be they beautiful misfires (Joss Whedon’s “Firefly”), bold misfits (James Gunn’s “Super”), experimental farce (Whedon’s “Dr. Horrible’s Sing-a-Long Blog”) or cosmic cartoons (he returns to the role of Green Lantern in the upcoming animated movie “Justice League: Doom”).

“When I go to a sci-fi convention, oh God, it’s the closest thing to being a rock star I will ever know in this life,” Fillion said over a coffee with an expression of rapturous deadpan. “I want to be a rock star, don’t you? It’s a good thing to be, a rock star.”

When you have won over cults as diverse as “Buffy the Vampire Slayer” and “Desperate Housewives” fans, for instance, you walk away with a special sort of celebrity. More than a million people follow Fillion on Twitter, and this past Valentine’s Day, readers of Entertainment Weekly voted his character, the charming boor Richard Castle, as the TV male they would most like to date.

It all adds up to a curious career odyssey for Fillion, who was raised by two English teachers in Canada and was himself on a path to become a high school teacher when his Edmonton theater work led in 1994 to a role on the soap opera “One Life to Live.” After a Daytime Emmy nomination, the soap job led to Los Angeles sitcom work on “Two Guys, a Girl and a Pizza Place” (which starred Ryan Reynolds, that other Canadian who wears a Green Lantern ring) and a brief but memorable role in Steven Spielberg’s “Saving Private Ryan” (playing a namesake for Matt Damon’s title character).

Nathan Fillion, center, played Capt. Malcolm Reynolds in "Buffy" creator Joss Whedon’s television series "Firefly." (Michael Lavine / Fox)

Fillion also landed the role of Caleb on the final season of “Buffy the Vampire Slayer,” a key early moment in his collaboration with Whedon. Fillion was Whedon’s leading man in the star-crossed sci-fi western series “Firefly” in 2002-03 and its feature-film sequel, “Serenity,” in 2005, and every few months there’s an industry rumor or a fan campaign calling for the franchise to saddle up again.

To the constituencies of Comic-Con International, Fillion feels like a homegrown superstar and he is acutely aware of his need to foster that part of his career. Last month, a “Castle” graphic novel (written by fan favorite Brian Michael Bendis) hit stores and also popped up as a carefully placed prop in a recent episode of the series. Fillion knows that “Castle” viewers who tune in to watch him and co-star Stana Katic solve murders represent a very different demographic than Comic-Con, but he says the line between mainstream sensibilities and genre tastes is narrowing.

“It’s so great in Hollywood now,” he said. “You have people past 40 sitting and talking about serious stuff, writing and making movies and TV, but there’s laser pistols and superheroes and alien monsters involved. It’s viable and mainstream. There’s a treasure trove of story, and film and television are dying for story, and comic books are like storyboards, and with special effect what can’t you do now?”

The 40-year-old Fillion longs to be in a big-budget sci-fi or superhero feature film but by no means is he overlooking the value of “Castle,” which started off as a big-city send-up of “Murder, She Wrote” and has morphed into a “Moonlighting” informed by “Law & Order” and “Bones.” The show is enjoying its best numbers ever right now.

Nathan Fillion, left, plays the show’s title character, mystery novelist Richard Castle, in the ABC series "Castle." (Patrick Harbron / ABC)

Like Hugh Laurie on “House,” Fillion clearly adores the cranky possibilities of playing a self-possessed scoundrel.

“I read him and immediately thought, ‘What a great time this would be to play him because he’s a kind of a jackass,’ ” Fillion said. “He’s doing his own thing, he’s selfish, he’s vain, he’s got all these character flaws. All of that I knew right away. What I didn’t know was how the mother and daughter relationship would humanize him. He’s not in control there, he doesn’t know how to be a dad and he gets humiliated a lot — that reduces him to something very basic that people can sympathize with.”

Fillion is never shy about mocking himself and that, according to Whedon, is the secret of his success.

“Nathan is a dork,” Whedon said. “He’s handsome, hilarious, a classic raconteur and a caring, considerate guy. But it’s his dorkiness, and his delight in it, that make it all more than charm. No one is more ready to poke fun at Nathan than Nathan. Except me.”

Nathan Fillion in the loft on the set of "Castle." (Anne Cusack / Los Angeles Times)

What’s next? Fillion is back with Whedon for the role of Dogberry in “Much Ado About Nothing,” a modern adaptation of the Shakespeare shot in black-and-white over 12 days in Santa Monica that earned cast members “hilariously miniature paychecks,” according to the press release. The film will be ready for the festival circuit on the other side of spring. It’s just another cult moment in a career oddly defined by fandom and fizzles.

“There were so many projects that I just loved — and that a lot of people loved — but they didn’t fly,” Fillion said. “You’re doing so many projects and then one makes it. And then you sit back and look at it and try to figure out why. Was it the timing or the mood? Is this what people were ready for at that moment? So many projects I’ve done fell short that I didn’t even imagine what it would be like to have a show reach Season Four. I’ve had the greatest time with the failures but this is OK, too.”