In Herbert’s novel, Thufir is captured by House Harkonnen when they invade the capital city of Arrakis and kill most of House Atreides. Although Baron Harkonnen already has a Mentat in his employ — the Twisted Mentat Piter De Vries — he forces Thufir to work for him. In the second half of the novel, the Harkonnens comb the desert looking for Paul and Jessica Atreides, the last surviving members of the royal family. Thufir stays close to the Baron and uses his position to sow discord between him and his evil nephew, Feyd-Rautha.
This subplot is cut from an admittedly already-ultra-dense story. Thufir might have been included, but if Villeneuve included every single detail from Herbert’s novel, his film would likely run 12 hours. Villeneuve wasn’t happy about the cuts he had to make, but he noted they were necessary, saying:
“When you adapt, there’s always some kind of violence toward the original material. […] You have to change things, you have to bend, you have to make painful choices. […] One of the most painful choices for me on this one was Thufir Hawat. […] He’s a character I absolutely love, but I decided right at the beginning that I was making a Bene Gesserit adaptation. That meant that Mentats are not as present as they should be, but it’s the nature of the adaptation.”
The lore of “Dune” is so complex that any film adaptation would have to make such cuts. Heck, Villeneuve already had to split the first novel into two films that, collectively, run five hours and 20 minutes. A film is not a book, and it must, by the nature of the medium, include fewer details than a 500-page novel might.
Sorry, Thufir.