Why it worked
The humor comes from the misunderstanding of Phoebe's career, escalating from a fan's admiration to the revelation of her pornographic film, creating a comedic and relatable situation for the audience.
Summary
A fan approaches Phoebe, asking for an autograph and praising her talent. Later, Joey is confused when Ross reveals that Phoebe is a porn star, showing a VHS tape titled "Phoebe Buffay in Buffay the Vampire Layer." The group decides not to watch it, despite some initial curiosity.