Jennifer Aniston broke co-star's rib on Office Christmas Party

Jennifer Aniston accidentally broke new co-star T.j. Miller's ribs after roughing him up during their very first meeting.

The pair plays brother and sister in festive film Office Christmas Party, but things didn't get off to a great start as the former Friends star was apparently unaware of her own strength during an early fight scene.

"The second I met Jen she immediately put me in a headlock," the Deadpool actor revealed at Entertainment Weekly's PopFest at The Reef event on Sunday (30Oct16). "And then (she) did a very strange pile-driver move, and my rib cage was broken."

Luckily, Miller was able to see the funny side of the odd encounter, adding of their onscreen bond, "So it was sort of an immediate chemistry, I thought!"

Office Christmas Party stars Aniston as a company CEO who tries to shut down a wild end-of-year bash at the Chicago branch of the firm, run by her brother.

The comedy also features Jason Bateman, Olivia Munn, and Courtney B. Vance, alongside 350 extras, who were asked by directors Josh Gordon and Will Speck to improvise as they filmed scenes for the festive bash.

However, two people took the advice a little too far and started pretending to have oral sex on camera - causing Bateman and Munn to do reshoots.

"We don't have much time to shoot these things," she explained.

Recalling the moment they spotted the inappropriate act in the playback footage, she said: "We come out and we shoot it four or five times. 'Great, let's move on. Wait... stop, stop, go back!'

"There was a couple, they're extras, and she was up on a filing cabinet and his head was between her legs and that's what they decided to improvise because they were told to do whatever they want!"

Office Christmas Party opens in theaters in December (16).