During Order of the Phoenix, Harry (Daniel Radcliffe) and his fellow fifth years find themselves frustrated and fed up when their new Defense Against the Dark Arts teacher, Professor Umbridge (Imelda Staunton), refuses to teach them any defensive spells. Spurred by Voldemort's (Ralph Fiennes) return, Harry and his fellow students form a secret organization called "Dumbledore's Army," where they practice spells in secret, and in the books, Harry's best friend Hermione Granger (Emma Watson) comes up with a clever way to ensure everyone's discretion. Hermione asks everyone to sign an official club ledger, and unbeknownst to everyone, she jinxes the list so that if anyone tells Umbridge about Dumbledore's Army, they'll be immediately identified. However, Hermione's jinx is left out of the film.
Ultimately, Cho's friend Marietta, who is apprehensive about Dumbledore's Army right from the start, tells Umbridge, revealing herself as the mole; immediately, her face is disfigured with a series of pustules that spell out the word "sneak," thanks to Hermione. Though Harry and his friends escape punishment, Marietta has to live with her shame, and Harry is furious with Cho for bringing her friend to the meeting in the first place.
However, in the films, without Hermione's jinx and Marietta, Cho is blamed in Marietta's stead, quickly revealed as the tattletale and dismissed immediately. Cho isn't the most likable character throughout the books, but she certainly didn't deserve Marietta's fate, especially when it would have been simple enough to feature another character.
The Harry Potter films are available to rent or buy on most major platforms.
