Nathan Fillion is a cult favorite actor, gaining a huge following after his portrayal of Captain Malcolm Reynolds in the short-lived Joss Whedon venture, Firefly.
Nathan Fillion is a fantastically talented character actor. From the first time he shows up on screen as Mal, you're drawn to the character right away. He's the renegade with the heart of gold, fighting against the ubiquitous space-government, The Alliance. Nathan Fillion makes Mal believable and you're always rooting for him as him and the crew pull the sometimes less than perfectly legitimate business enterprises. Mal is one of the most layered characters in recent memory, and Nathan's performance as him is something to form a precedant for truly fantastic acting. He retains that Firefly is the most fun he's had acting, and he's still friends with all of the cast.
Aside from Firefly, his other forays into tv and film are also worth noting. He had recurring characters on Two Guys, A Girl and a Pizza Place and Drive, as well as cameos on such shows as Lost, Desperate Housewives, One Life to Live and, another Jossverse show, Buffy the Vampire Slayer. As far as movies, one of his most notable role were as Bill Pardy in the James Gunn horror homage flick Slither. His role in Slither is one of the more accurate portrayals of a real person in the situation of an alien zombie movie, with the appropriate levels of freaking out and not having his plans go smooth. He was also in the indie romance movie Waitress, and even so far back as a role in Saving Private Ryan.
He remains a geek icon thanks to some of his dabblings in video games and animation. He leant his voice to, fittingly, a Sargeant in Halo 3, as well as working with Bungie again later as Buck in Halo ODST. He also voiced Vigilante in the Justice League cartoon, alongside his fellow Firefly alumni Adam Baldwin (Hal Jordan, The Green Lantern), Gina Torres (Vixen) and Morena Baccarin (Black Canary). He was in the Wonder Woman animated movie as well, voicing Steve Trevor.
He also joined up with Joss Whedon again during the writer's strike when he acted in his musical Internet venture of Dr.Horrible's Singalong Blog next to Neil Patrick Harris. It's then we find out that, holy shit, he can sing! Dr.Horrible was more than welcome during the creative lull of the strike, and became so acclaimed, there is talk of a sequel.
Nathan now enjoys the success of ABC's police drama/comedy Castle as the lead role, as well as the impending release of another James Gunn film, Super, where he acts alongside Rainn Wilson, Ellen Page, Liv Tyler and Kevin Bacon. Here's hoping his success continues in the years to come.