Nobody
The film was never cast, because it was never bought

Scream’s Stab Movies
UNMADE
It's all about true-crime limited series these days.
There is no ninth Stab film. The Stab movies were based on Gale Weathers' books. Per the Scream Wiki, the first three followed the real Ghost Face murders and the later ones were made up. Stab 8, directed by Rian Johnson and released as just Stab, is the last one that got made.
The killers in Scream (2022) wanted a new Stab. Richie Kirsch and Amber Freeman hated Stab 8. They staged real murders in Woodsboro to give Hollywood new source material, so the next Stab would go back to basics and be based on actual events. When it was over, Gale said they could die in anonymity. She said she might write about a good man who used to be the sheriff instead, meaning Dewey Riley.
By Scream VI, a year later, Gale has gone back on that and written the book anyway. Her line is that somebody was going to write about it, and writing is what she does. Sam Carpenter confronts her. Sam says the book called her unstable and a born killer. Gale says that was taken out of context. The answer comes back that it is literally a quote.
In the same fight comes the jab: "I heard you couldn't sell the movie rights." Gale brushes it off and says it is all about true-crime limited series these days. No studio bought the book, and no new Stab film is named after that.
The book's title is never said in the film. It showed up at the Scream VI Experience in Santa Monica before release: Requel: Terror Returns to Woodsboro. The Scream Wiki lists it as Gale's latest book.
The closest thing to a ninth Stab is Richie's own work. In Scream VI Detective Wayne Bailey says Richie really loved the Stab movies and even made a few of his own. During the final fight in the shrine, Richie's homemade Ghost Face video plays on a projector in the background, per the Scream Wiki.
Scream 7 (2026) centers on Sidney Prescott again. The Stab movies still come up in its dialogue, and when someone is asked for a favorite scary movie the answer is Stab One, but the film names no new Stab and says nothing about Gale's book becoming a film.
The film was never cast, because it was never bought
Scream VI opened March 10, 2023, from Paramount Pictures. The unmade ninth Stab exists only as the lines about Gale's book and the movie rights.
The book title came from the Scream VI Experience, a walk-through promotion in Santa Monica, California. Perri Nemiroff reported it for Collider on February 26, 2023. A bodega set there held Gale's earlier books The Woodsboro Murders and Hollywood Horror, Sidney Prescott's book Out of Darkness, and a new one, Requel: Terror Returns to Woodsboro.
On the Scream VI commentary, as summed up by Queer Horror Movies, the directors say Richie's homemade movie was shot on weekends by their assistants Adam Stand and Adriana King, and King is the girl in it. The title lettering borrows the font from the first two seasons of Twin Peaks. Jack Quaid came back to shoot inserts for the film, such as pictures and phone images.
No ninth Stab was produced, so there is no composer, score, soundtrack, theatrical or streaming release, or digital, VHS, DVD, Blu-ray or 4K edition. Gale's unsold movie rights in Scream VI are a story point, not a missing release.
Requel: Terror Returns to Woodsboro, a prop book on the Scream VI Experience bodega set, 2023.
The Scream VI shrine shows Richie's personal copies of Gale's books. The Scream Wiki says it is the first time her first book, Wrongly Accused, is seen on screen.
A projector and white cloth in the shrine, used to show the Stab movies and Richie's videos, per the Scream Wiki.
Richie's homemade Stab film, per the Scream Wiki (unconfirmed details): about 18 minutes, with a cheap redo of Casey Becker's murder, Richie talking to the camera between scenes, a girl stabbed in the back behind bars, and Richie showing off his masks before one is burned.