TJ Miller reveals Jennifer Aniston broke his ribs on set of Office Christmas Party

She's been doing glamorous appearances promoting upcoming flick Office Christmas Party.

But it appears things weren't quite as smooth behind the scenes, as co-star TJ Miller has revealed Jennifer Aniston accidentally broke his ribs on set of their new film.

Speaking at Entertainment Weekly's PopFest at The Reef on Sunday, the 35-year-old actor revealed the 47-year-old Friends star put him in a headlock as soon as she met him, injuring him by mistake.

"The second I met Jen she immediately put me in a headlock," he said.

"And then did a very strange pile-driver move, and my rib cage was broken. So it was sort of an immediate chemistry, I thought."

The pair play brother and sister, Carol Vanstone (Aniston) and Clay (Miller), in the movie which sees the Friends star's CEO alter-ego attempt to close down the Christmas party at the Chicago branch of her company, which is run by her sibling.

But it results in the staff staging a wild bash to try to impress a potential client.

The movie featured 350 extras who were told to improvise throughout the scenes, but one pair took the instruction too far by mocking oral sex, resulting in Olivia Munn having to re-shoot part of the film.

She explained: "We don't have much time to shoot these things. 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 improve because they were told to do whatever they want."

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But Miller insisted the extras were the "most important part" of the motion picture because they had to film countless dancing scenes without music.

Speaking at Entertainment Weekly's PopFest in Los Angeles, he added: "I want to be very clear that the background artists were the most important part of this film.

"They were dancing in the background, with no music, for 12 hours, a day, for weeks and weeks. Just silence."

Office Christmas Party, which hits NZ theatres on December 8th - marks the fifth on-screen collaboration between the Friends alum and co-star Jason Bateman.