The British actor Gary Oldman is apparently "in negotiations to join the cast" of the flick.
That's according to movie website The Hollywood Reporter who have said Oldman is up for the role of the mysterious and scary Sirius Black.
Sirius escapes from Azkaban, the wizard prison, in book three. And the whole wizarding world begins to fear he is coming after Harry Potter!
Gary Oldman has been in many films, including Lost in Space, The Fifth Element and Bram Stoker's Dracula.
Warner Brothers, the film's makers, told the Newsround website that they weren't commenting on the rumours.
But we'll let you know when we find out whether it's true or not!