Justin Timberlake and Mila Kunis
04/27/2011 at 01:00 PM EDT
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.
"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