Answer by taking a quote from the following article:

"Trapped in the Closet" is the twelfth episode in the ninth season of the American animated television series South Park. The 137th episode of the series overall, it originally aired on Comedy Central in the United States on November 16, 2005. In the episode, Stan joins Scientology in an attempt to find something "fun and free". After the discovery of his surprisingly high "thetan levels", he is recognized as the reincarnation of L. Ron Hubbard, the founder of the church.

Saving for a bicycle and not wanting to spend money on entertainment, Stan takes a free "personality test" being offered by Scientologists on the street. After answering a long questionnaire, Stan is informed that he is extremely depressed and therefore a perfect candidate for Scientology. They offer to help him out for $240. Back home, Stan asks his parents for the money. His father suggests that he use the money he had been saving. Stan pays and is taken into an auditing room where an attendant reads his "thetan levels" using an "E-meter". Stan has such a high reading that the Scientology headquarters in Los Angeles is notified. There, the president of Scientology determines that, because his reading is so high, Stan must be a reincarnation of L. Ron Hubbard, Scientology's founder and prophet.  Later that night, a large group of Scientologists, including John Travolta, gather outside the Marsh house to celebrate Hubbard's "second coming". The president of Scientology arrives in a helicopter and talks with Stan's parents. They are opposed to Stan's participation, but the president informs them that "we're not asking him to join us; we're asking him to lead us". Randy sends Stan to his room, where he finds Tom Cruise waiting. Cruise, believing that Stan is genuinely Hubbard's reincarnation, asks him whether he has enjoyed his acting. When "Hubbard" replies that his acting is okay but not as good as others' such as Leonardo DiCaprio or the Napoleon Dynamite guy, Tom hears that he is "a failure in the eyes of the prophet" and locks himself in Stan's closet. He refuses to come out, despite the entreaties of Randy, Nicole Kidman, the police, Travolta and R. Kelly to "come out of the closet". Travolta and Kelly eventually join Cruise in the closet.  Downstairs, the church president tries to convince Stan's parents to allow their son to participate. He tells to Stan the great secret behind the church -- a condensed version of the story of Xenu, based directly on the Scientology Operating Thetan III document, and accompanied by an onscreen caption reading "This is what Scientologists actually believe". He then begs Stan to continue writing where "L. Ron" left off. Stan, impressed by the story, does so. He shows his writings to the Scientology president, who initially approves of the work, but when Stan says "to really be a church, you can't charge money to help", the president reveals to Stan that the church is in reality a global money-making scam. He asks that Stan continue with that in mind. Stan appears to agree and keeps writing.  Outside the house, the president introduces Stan to his followers, to whom he will read parts of his new doctrine. However, instead of presenting it to them, Stan states that he is not the reincarnation of L. Ron Hubbard, and that "Scientology is just a big fat global scam". The Scientologists grow angry and threaten to sue him. The celebrities in the closet appear, threatening to sue Stan as well. The last shot is Stan daring them to do so. However, that shot is followed immediately by the closing credits naming only "John Smith" and "Jane Smith", a reference to Tom Cruise and the Church of Scientology's reputation for litigiousness.

Were any famous people characters?
Later that night, a large group of Scientologists, including John Travolta, gather outside the Marsh house to