Kristin Davis
You know her best as Charolette from Sex and the City, but she also played Joey's love interest on Friends! Apparently, despite being at the peak of her fame, she was a nervous wreck appearing on the show. Thankfully, she pushed past the anxiety for a stellar performance.
Sean Penn
Sean Penn was really out of this world in this episode of Friends, huh? The actor appeared several times on the show, but not as the main character or even a love interest. He was the boyfriend of Phoebe's twin sister Ursula. Known to be sweet and gullible, he was even her fiancée.