Nathan Christopher Fillion (born March 27, 1971) is a Canadian actor, known for his lead role as Captain Malcolm Reynolds in the television series Firefly and its film continuation Serenity, Caleb in the final five episodes of Buffy the Vampire Slayer, voice acting for Halo 3 as Gunnery Sergeant Reynolds, and Halo 3: ODST, Halo: Reach, and Halo 5: Guardians as Edward Buck, and his current lead role as Richard Castle on the ABC series Castle. Nathan is also known for playing Captain Hammer in Dr. Horrible's Sing-Along Blog.