Justin Timberlake & Jessica Biel Together at His Premiere Party

By Jessica Wedemeyer and Liz Raftery

10/21/2011 at 08:45 AM EDT

Justin Timberlake and Jessica Biel have remained friendly since their breakup earlier this year – but now it seems their romance may have been rekindled beyond bike rides.

The off-again, on-again couple attended the premiere after-party of Timberlake's new movie, In Time, in Los Angeles Thursday, and stole a few intimate moments together, according to an onlooker.

They weren't attached at the hip the entire time – Timberlake, 30, greeted guests at the event while Biel, 29, hung out with a small group of friends and at one point picked up some tacos from a food truck – but they stayed at the party past midnight, after most of the other guests had left.

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"[They] became more and more affectionate, sneaking kisses, whispering to each other, and sharing quick back rubs and caresses," the eyewitness said. "At one point, she pretended to punch him a couple times and he gamely bounced up and down like a jack in the box."

Timberlake's costars in the movie, which opens Oct. 28, Olivia Wilde and Amanda Seyfried, also mingled with guests after the premiere.