Marci Maven (played by Sarah Silverman) was an obsessive fan of TV star Brad Terry, who was constantly hounding him with attention, and suggestions for improving his show. She followed him to the house of his ex-wife, Susan Malloy on the morning that she was murdered. Because of her obsessive attention to every detail of Brad's life, she recognized a clue that proved Brad was the murderer, but, in her devotion, chose to confess to the murder herself rather than betray him. Given her nutty behavior, few people had trouble believing that she had killed Susan, to remove her from Brad's life.
However, Adrian Monk was one of those few, and exposed Brad as the culprit. Marci immediately abandoned her devotion to Brad, and transferred it to Monk. Her attentions eventually forced Monk to request a restraining order that prevented her from coming near him. Undeterred, Marci started a Monk fansite on the Internet, composed songs to him on her guitar, and amused herself building dioramas of Monk's various cases. She also stocked her wardrobe and kitchenware with articles taken from his garbage (since he threw clothes and dishes out so frequently).
When Marci's neighbor, John Ringel, framed her recently-deceased dog, Otto, for the murder of Ringel's wife, she called on Monk for help. Since he refused to let her near him, she resorted to buying a date with him at a SFPD bachelor auction. Despite himself, Monk became interested in her case.
Marci had secret fantasies of replacing Natalie as Monk's assistant, and during their brief partnership, Monk couldn't help but be impressed that Marci knew the catalog of his phobias and preferences backwards and forwards. Natalie, more cynically, said that Monk, who was "only human," couldn't resist the attentions of anyone who thought he was a god.
However, Marci abandoned her interest in Monk when she got a taste of what it was really like to be a detective's assistant: receiving a non-fatal gunshot wound from Ringel, and being taken hostage by him before she was rescued by the police. She sent all her memorabilia back to Monk, saying her new idol, actor F. Murray Abraham, "wouldn't understand." Monk, tremendously relieved to be rid of Marci, said a prayer for Abraham. Despite this, however, she did seem to keep enough of her Monk collection, as evidenced by her displaying it to John Novak in his In Focus special on Monk's 100th case.
Marci was played by Sarah Silverman. Between the time of her two appearances, Season Two's "Mr. Monk and the TV Star" and Season Six's "Mr. Monk and His Biggest Fan," Silverman's career took off with the production of her own show. There was some doubt among the show's producers about whether she would be willing to play a guest star again for the second episode, but Silverman was actually delighted to do so.