He has co-written several of director Terry Gilliam's films, including Fear and Loathing in Las Vegas and Tideland. Gilliam and Grisoni went to WGA arbitration to get credit for Fear and Loathing; initially only Tod Davies and Alex Cox (who had written a previous version of the screenplay) were credited, despite Gilliam and Grisoni rewriting the entire film from scratch. In the end, all four writers were credited. Later, Gilliam and Grisoni were involved in a similar dispute when original writer Ehren Kruger received sole credit for The Brothers Grimm; as an alternative to receiving a writing credit, Gilliam and Grisoni listed themselves as "dress pattern makers".
Grisoni also co-wrote the screenplay for Gilliam's The Man Who Killed Don Quixote, which was abandoned soon after starting filming. During the production, he met filmmakers Keith Fulton and Louis Pepe, who were shooting the making-of documentary that later became Lost in La Mancha. Fulton and Pepe went on to film Brian Aldiss's novella Brothers of the Head (2006), from a screenplay by Grisoni that he had started working on as far back as 1984.
In 2008, Grisoni directed his first short film, Kingsland #1: The Dreamer, for which he was nominated for a BAFTA.

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