Justin Timberlake: Friends with Benefits Is a Good Idea ... Until It Isn't

04/27/2011 at 02:15 PM EDT

Justin Timberlake plays a guy in a casual relationship with his friend in the new movie Friends with Benefits – but the singer-actor isn't such a fan of casual hookups in real life.

"It's a really good idea until it becomes a bad idea," Timberlake tells Ellen DeGeneres in an interview with his costar Mila Kunis scheduled to air Thursday. "It probably becomes a bad idea really fast."

Timberlake, 30, who recently split from longtime girlfriend Jessica Biel and subsequently denied any romantic relationship with Kunis, says it's hard to keep emotions from interfering – especially when sex is involved.

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"If you're going to be intimate with someone at some point somebody's going to feel something," he says.

Jokes Ellen: "If it's bad, then no one's going to develop any feelings."

"They'll develop a feeling," says Timberlake, "but it's going to be uh, uh."