Matt Damon and Heath Ledger were originally cast in opposite roles. They petitioned and switched their roles.
Because of problems with the Writers Guild of America, Terry Gilliam and Tony Grisoni were not able to credit themselves as writers of the screenplay, despite the many changes they made to Ehren Kruger's original script. They invented a credit for themselves as "Dress Pattern Makers" and were quoted as saying that the film was made from a "dress pattern," not necessarily made a "screenplay."
Historically, Jacob Grimm was older than his brother Wilhelm Grimm. Jacob was born in 1785 and Wilhelm in 1786.
The glass slippers are from Charles Perrault's original French version of Cinderella. (In fact they were fur slippers, later changed to glass by a typo because the words are very similar in French.) In the Grimm adaptation, she wears gold slippers.
Terry Gilliam: [cats] Gilliam's tradition of brutalising cats for laughs (see Die Ritter der Kokosnuß (1975)) is continued by Cavaldi's encounter with the kitten in the dungeon.
Terry Gilliam: [burst] People and things "burst" out at the camera, e.g., the first witch the Brothers "exorcise."