Last month, Julia Roberts surprised the audience at Taylor Swift's "1989" concert stop in Santa Clara, Calif., when she and Joan Baez walked out onstage with Swift. Roberts danced her way down the stage to the pop star's hit song "Style" like many other of Swift's famous friends did before.

Roberts revealed to Extra the story behind her appearance backstage at the Givenchy New York Fashion Week Show and said it was like an out-of-body experience. The Oscar winner took her three children to see Swift's show and met the "Bad Blood" singer backstage before the concert. Swift asked the actress backstage if she would mind coming onstage with her and dancing. Nervous about what to do, Roberts asked the opinion of her children.

"'Oh Mom, you have to do it'" Roberts said one of her kids told her. "And my youngest son said, 'Only do it if you want to, Mommy.' It was so sweet!"

When it came time for Roberts' surprise appearance, she said she was very nervous.

"It was one of those crazy out-of-body moments. I can't tell you anything that happened from the moment I came on stage," she said.

Roberts also revealed that the "1989" concert was her kids' first concert ever! It must have been one to remember!

Check out Roberts as she talks about the appearance below!

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