Smith will again play heart surgeon Erica Hahn on the popular prime-time show.
"I've been dabbling over there in Season 2 and then in Season 3," Smith said. "After every time, I would hope that they would call and also hope not, too -- kind of like when I used to be single. And they finally, actually called."
Smith told The Times she already knew most of the cast from past projects.
"I haven't met anyone who's not nice there," she said. "I did a movie with Patrick (Dempsey), and I did a play with Sandra (Oh) in New York called, 'Stop Kiss.' We were lovers. ... When I first got to 'Grey's,' she told the producers I was a good kisser. Ooh, very controversial. And I did a 'Law & Order' a million years ago with Ellen (Pompeo). And T.R. (Knight) I met T.R. at the O'Neill Theater Festival. Very highbrow. What can I say?"