Aside from the dazzling costumes and pitch-perfect setlist, the thing we’ll always cherish about Taylor Swift’s 1989 World Tour was the star-studded roster of celeb guests, which added an element of surprise that Swifties everywhere reveled in.

With so many famous faces taking the stage each night, you'd think each appearance was meticulously plotted weeks or even months in advance. But thanks to Julia Roberts, we now know that Tay sometimes waited until the very last minute to concoct her celeb guest magic.

During an interview with Extra TV, Roberts spilled the story behind her induction into the Swift Squad, which happened at Taylor’s tour stop in Santa Clara, Calif., on Aug. 15. When asked if her concert cameo was planned ahead of time, Roberts said, “No! Oh my God, I would have washed my hair if it had been planned. I was wearing a ponytail, leggings, and tennis shoes.”

The actress said she took her three kids — twins Phinnaeus and Hazel, 10, and Henry, 8 — to the concert (their very first one!), where they met Taylor backstage before the show and the deal went down.

“She said, ‘Would you mind coming on stage during the song and dancing?’” Roberts remembered. She consulted with her kids, who told her, “Oh, Mom you have to do it!”

And even though there was no time to shampoo her tresses, she looked every bit a “Pretty Woman” when she and legendary folk singer Joan Baez burst onstage, outfitted in Taylor Swift tees. Together, they strutted down the runway during “Style” on either side of the woman of the hour, looking positively ecstatic.

But don’t let Roberts’ famous mega-watt smile fool you — she admitted to being “so nervous” onstage in front of tens of thousands of screaming fans, admitting she essentially blacked out from the sheer excitement.

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

Don’t worry, Julia — that’s what YouTube is for: