Over its decade-long run, Friends guest stars included some of Hollywood’s biggest names, yet many of them were reportedly terrified to appear on the show. Across a ten-year stint, Friends became critically acclaimed, with 62 Emmy nominations in total. It was also a cultural phenomenon and trend-maker, filling the mid-90s and early 2000s with slip dresses, “Rachel” cuts, and phrases like “Friend zone.” Friends was television gold—yet one essential element of the series terrified even the biggest of Friends guest stars.

When Friends aired on September 22, 1994, it premiered after NBC’s Mad About You and before Seinfeld. Sandwiched between two hit series with established fan bases that loved watching New Yorkers be New Yorkers, Friends was an instant hit. In its freshman outing, Friends season 1 guest stars included George Clooney, Noah Wyle, and Jennifer Grey. From there, the Friends train only picked up speed with mega-stars like Tom Selleck, Julia Roberts, Charlie Sheen, Isabella Rossellini, Ben Stiller, Robin Williams, Billy Crystal, and Charlton Heston all appearing on the show before the turn of the century. In its final four years, Friends guest stars also included Susan Sarandon, Bruce Willis, Denise Richards, Wynona Ryder, Brad Pitt, Freddie Prinze Jr., Selma Blair, Jeff Goldblum, Greg Kinnear, Danny DeVito, and Dakota Fanning, to name a few.

Related: Friends: The True Story Behind Brad Pitt’s Cameo

Yet, despite many of the guest stars’ big-screen status, Friends actress Jennifer Aniston recently told Pam & Tommy’s Sebastian Stan (via Variety) that most of the series’ guests were “terrified” to be on the show. The actress claims that the sitcom’s live audience spooked the box-office actors—something Stan readily agreed would terrify him. And while viewers barely register the frequent laughter and reactions from the series’ audience, it makes sense that actors unaccustomed to a live audience would find the experience daunting.

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A live, in-studio audience is truly an environment all its own. Unlike theater-trained actors accustomed to an audience setting, filming with a live, in-studio audience means acting for a camera and a crowd. Similarly, film actors are accustomed to filming in a closed-set environment where the camera is the audience. Adding a live audience means that actors experience the immediate reaction of the viewers—good and bad.

It was not only the Friends guest stars who were intimidated by the live audience. Matthew Perry, who played Chandler on Friends, confessed (via Independent) that he too was terrified: “To me, I felt like I was going to die if they didn’t laugh… I would sweat and just go into convulsions if I didn’t get the laugh I was supposed to get. I would freak out.”

In addition to Perry’s point, it stands to reason that film actors would find it challenging to accommodate audience reaction times naturally in their performance. Another difficulty might be remaining in character despite the audience’s reactions. Considering the incalculable variables involved in a live-audience setting, this makes all the guest performances on Friends even more impressive.

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