United Kingdom, June 4 -- Jennifer Aniston couldn't believe how "nervous" Hollywood stars were on Friends.

The 57-year-old actress, best known for playing Rachel Green on the iconic sitcom, has recalled the show's string of famous guest stars, which included her now-ex Brad Pitt.

Speaking with co-star Lisa Kudrow, 62, for Variety's Actors on Actors series, she said: "The movie stars that came on Friends. Brad, Bruce Willis, Julia Roberts. Isabella Rossellini.

"We had so many - Sean Penn. They were always nervous. Remember? I always found it fascinating how nervous they were."

Lisa, who played Phoebe Buffay on the show, thinks the performance style on the show was different to the film and theatre worlds.

She explained: "Yes. Nervous....