Although the Friends reunion special had plenty of time for the show's six leads to reminisce about the series, the recently released HBO Max special also spends plenty of its run time on appearances from celebrity guests. Everyone from BTS to Malala Yousafzai makes an appearance, as does Lady Gaga, who gets to do a duet on "Smelly Cat" with Lisa Kudrow.Inside Lady Gaga's appearance at the 'Friends' reunion.About an hour into the reunion special, Lisa begins playing "Smelly Cat" on her guitar on the famous Central Perk set, only to be interrupted by Lady Gaga, who enters the set carrying her own guitar. "I love 'Smelly Cat,'" the singer said. "It's one of my favorite songs."\n\nGaga then proceeded to perform the song's chorus before the two united for a duet to close things out.After Lisa and Gaga finished with their performances, the Star is Born star spoke about what the character of Phoebe meant to her growing up. \n\n"Thank you so much for being the person for all of us on Friends that was the, I don't think if this is the right way to say it, but the different one or the one that was really herself," the singer said.Was Lady Gaga on 'Friends'?Although she played a role in the reunion, Lady Gaga was never on Friends during its original run. Gaga was born in 1986 and would have been 8 when the show premiered and 18 when it ended. While there were plenty of roles for young actors on the show, Gaga didn't play any of them. She wouldn't emerge into the world of pop music until well after the show had aired its final episode.In speaking with Variety, the show's director Ben Winston discussed the decision to include Gaga in the reunion. He explained that Lisa was open to playing "Smelly Cat", and they both agreed that having Gaga join would take the performance up a level because of how open the singer has been about what Phoebe meant to her as she was growing up.\n\n"That was a really beautiful moment," Ben said. "And Lady Gaga jumped at the chance to do it."The 'Friends' reunion featured plenty of familiar faces.Although every guest star who was featured on the show wasn't available for the reunion, the special did feature plenty of familiar faces for fans of the show. Reese Witherspoon, who played Rachel's sister on the series, appears in interviews, as does Maggie Wheeler, who played Chandler's on-and-off girlfriend Chandler on the series.The special also featured a couple of interesting revelations about the series, including the fact that Jennifer Aniston and David Schwimmer, who played Ross and Rachel on the show, had a crush on each other in real life as well. The full special also features plenty of more general reminiscing, and the cast also returns to the set where they filmed so much of the series. The special debuted on HBO Max on May 27 and is available in full for all subscribers.