The episode in question is during the seventh season of Friends.

Rachel deals with her apprehension for babysitting Ross’s son Ben by teaching him practical jokes.

She instructs Ben how to pull off a few simple pranks, including a quarter trick that leaves marks on people’s faces and plastic wrap on a toilet seat.

Ross becomes upset over the pranks and tries to make Rachel talk Ben out of doing them. But she remains insistent that she likes being “Fun Aunt Rachel.”

When Ross chases Ben out of the apartment and up the stairs of the apartment building. 
Rachel follows and witnesses what she thinks is Ross’s real body falling down the stairs. She screams in horror only to find it was a dummy wearing similar clothing.

Aniston pulls off a convincing scream when the dummy gets thrown down the stairs. That’s because she never knew about the prank in the first place and thought Schwimmer truly did fall.

According to Wiki Fandom, this is a common tactic that directors use to capture true emotions from the actors. And in Aniston’s case, it worked like a charm.