Leigh's character Lexie ended up in the shower with Jackson (Jesse Williams) during the last episode.
Speaking to Entertainment Weekly, Leigh explained that when the show returns the pair will have been dating for a while.
"A few weeks have gone by, and in this episode coming up this Thursday, you see more of a development between Lexie and Jackson," she said. "Lexie is able to kind of just enjoy herself and it's refreshing - very 'no strings attached'."
She added: "They're in the same place in their lives. They want the same thing. So what starts out as something that seems kind of a bit flingy, turns into a bit of a relationship. Nobody is dropping the 'L' word or anything, but you start to see a little bit more of a development and potential that something could really happen there."
However, Leigh revealed that Mark (Eric Dane) "doesn't necessarily react so well" when he finds out about the romance.
"When it comes down to it and when you strip everything else away, Mark and Lexie still love each other," she said. "[But] that's not where she is. So it's definitely a difficult situation between Lexie and Mark."
Grey's Anatomy airs on Thursday at 9/8c on ABC.
> 'Grey's' star promises romance struggle