According to Life & Style, the two began casually seeing one another recently and have decided to take things slowly.
"They've been going out on dates in NYC," a source said.
"It started as just a hookup, but now they're dating. Sebastian doesn't want to make the same mistakes as with [ex-girlfriend] Leighton [Meester]. She was so public, he's more focused on his own career right now and wants to just have fun with Dianna without it being this big thing."
Stan can next be seen alongside Natalie Portman in Black Swan, to be released later this year.