Rivers looks so much like her late mother Joan that she told People the experience was emotional.
"I think I was more freaked out in the hair and makeup tests than I was once I got on set," she explained. "It was bizarre. I stopped looking in a mirror."
She added that the full effect was "quite something."
The David O. Russell-directed film stars Jennifer Lawrence and Bradley Cooper. It follows the real-life story of entrepreneur Joy Mangano. Rivers will appear in a small part of the film during a scene that takes place at a taping of Joan's QVC show.
Rivers said the cast and crew couldn't have been nicer or more supportive, especially one star in particular.
"My mother had a couple lovely encounters with Bradley, and she was a fan of Jen's even though there was a situation once at Fashion Police where someone said something and Jennifer didn't like it and kind of fired back," Rivers said. "It wasn't my mother who said it and it kind of got lost in translation. The first thing Jennifer said was, 'Even though we had that disagreement that really had nothing to do with her, I was a huge fan,' and quoted back her favorite Fashion Police line ever to me: 'That dress is so ugly it looks like it would be good at math.' So here she is quoting my own show back to me, then she said to David, 'I'm so happy you brought me someone to play with!'"
People also asked Rivers how she thought her mom would react to her role in Joy, and her response was perfect.
"I think she actually was the voice in my head of, 'You get to work with these people?! Run, don't walk,'" she explained with a laugh.
Joy will be released Dec. 25.
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