David Boreanaz
Boreanaz starred in Buffy, if you didn't know that already. He always thought that he had a face for the camera, and one day when he was simply out walking his dog, he got the break of a lifetime...
Keep It in the Neighborhood
Boreanaz happened to be unemployed at the time, and he struck up a conversation with his neighbor, who just so happened to be friends with Marti Noxon, the writer of Buffy the Vampire Slayer. Boreanaz's neighbor said that the role needed to be filled by an impossibly handsome man, and he offered him an audition. The rest is history.