From PEOPLE Magazine Click to enlarge
As Richard Castle, the wisecracking mystery writer on the hit ABC drama Castle, Nathan Fillion handily helps the NYPD solve crimes each week. But in real life the actor can't seem to crack a caper that's taking place in his own backyard: A mole has infiltrated the perimeter of his L.A. home, and "it's tearing my lawn apart," Fillion laments. "I don't want to kill the cute little thing, but it's just not leaving."

That furry critter isn't the only one who's becoming a permanent fixture in Hollywood. After a decade of failed series and building a cult following in sci-fi shows like Firefly, the actor is finally a bona fide TV star-and heartthrob-thanks to Castle, now in its third season. Fillion says he's "very appreciative," of his newfound fame but adds that he's trying to keep it in perspective. "I know things can't always last, so I'm grateful for when things go well-and I'm planning for when things are not."

Despite turning 40 on March 27, Fillion still has the same exuberance for comic books, science fiction and animation that he did as a boy in Edmonton, Alta. "My mom says, 'Nathan, you're very much a geek, but you look mainstream,'" says Fillion, who spends many nights playing Xbox with pals and enjoys attending sci-fi conventions. " 'Nobody can tell by looking at you. But if they hang around with you 10 minutes, you're going to spill it.'"

Growing up, Fillion aspired to be a teacher like his parents, Bob and Cookie. He enjoyed acting but went to college "as a backup." In 1994, four months shy of graduating from the University of Alberta with an education degree, Fillion was cast as Joey Buchanan on One Life to Live. Three years later he moved to Los Angeles-where he nearly threw in the towel after almost a year of failed auditions. "I was paying my rent on credit," he recalls, "thinking, 'Have I made a colossal error of judgment?'" But then the gigs starting coming, including supporting parts in Two Guys, a Girl and a Pizza Place, Buffy the Vampire Slayer and Desperate Housewives, on which he played Dana Delany's ex-husband.

When Castle came along in 2009, it cemented Fillion's sexy leading man status, though he tries to refute the label. "People see me on TV playing likable roles," he says. "If you get close enough to anybody, they'll get ugly. I have a big nose. It's crooked. I have crooked teeth." Crooked or not, his Castle costar Stana Katic, who plays his partner Det. Kate Beckett, remains charmed: "He's a spectacular guy," she told PEOPLE last year. "He's like an older brother-and so funny!"

Despite his big-kid indulgences, Fillion, who says he has a "lovely girlfriend" he declines to name, insists that marriage is definitely in his future. "My parents are still in love-they married in 1967," he says with admiration. "I'm very traditional like that."

And certain traditions remain sacred to him, like the one he shares with his mother each year on his birthday: "My mom will tell me the story about the day I was born," says Fillion. Now, thanks to finding mainstream success, his recent 40th birthday brought him a long-awaited gift: contentment. "I like where I am. I love my house. I have a very good job," he says. "Now is a neat time."