In the case of Dune, however, there have already been two filmed versions; David Lynch's 1982 bomb and the somewhat-more-successful 2000 Sci-Fi Channel mini-series. Of the two, the mini-series was marginally better, at least in getting across the ideas in the novel, which the movie tried to compress into just over two hours, and, like a lot of movies based on long novels, poured a ton of detail into the first hour and then rushed the second.
I think Dune is pretty cinematic, and since its sequels mostly feature different characters and lots of time-jumps, I think it fits much better as a movie. Perhaps it could be turned into a six-hour "Event" television series, or perhaps it could follow Kill Bill and Harry Potter and the Deathly Hallows and just split into two (three hour plus) movies. I'm good with either option, and it doesn't change my casting choices either way.
The role of Duke Leto Atreides, Paul's father, Jessica's lover and, at the start of the story, the Duke of Caladan and also of Arrakis (the desert planet known colloquially as "Dune"), is not a huge part, but is very important and memorable, and will require a solid, recognizable actor in the part. Since he is nobility, he will have to look high-born and noble, but since he's a good man, it's important he also be likeable. I admit, I struggled with this one. None of the movies got the Duke right before. I struggle between Jason Isaacs and Mark Strong, with Strong ultimately winning out.
The story's principle villain is Vladimir Harkonnen, the despicable Baron of the planet Geidi Prime, who is the other half of an ages-long feud between his house and House Atreides. He is secretly in league with the Paddishah Emperor and the Spacing Guild to destroy House Atreides. He is described in the book as being grossly fat, to a degree that he needs an anti-grave belt to even stand up, and speaking in a rumbling "basso" voice. He had red hair as a young man, but that's not important to the character, plus you could even make him bald.
Neither film version really cast him well. They just went for a heavy-set actor. Baron Harkonnen is supposed to be almost inhumanly fat. After going through a number of fat actors such as Pruitt Taylor Vince, Timothy Spall and Simon Fisher-Becker, I decided that it was far more important that the actor chosen have villainous gravitas and an evil look than that he be overweight. After all, there's probably no one outside of Guinness World Champions who would be fat enough, so I decided to choose a creepy, evil looking actor and let a thousand pounds of latex (like so) take care of the rest. I chose Hugo Weaving.
The final Atreides retainer is swordmaster Duncan Idaho. Idaho is not a large role, but he's a fan
While we're casting the second-tier villains, let's look at the other half of the anti-Atreides conspiracy, the Paddishah Emperor Shaddam IV. The character doesn't actually appear on-page much in the novel, but is nonetheless a pretty central role who needs to be featured. He's described as looking much younger than his eighty-plus years, with red hair. I was watching Doctor Who when I saw the perfect Emperor; Scottish ginger-haired middle-aged character actor Tony Curran.
Poots seems a good fit for the role.
Moving on, we have creepy Mentat Piter DeVries, creepier eunuch Count Hasomir Fenring, and hulking brute (and Feyd-Rautha's half-brother) Count Glossu Rabban. These are sorta the henchmen roles. Piter is the Baron's personal Mentat, and is described as having a "killer's smile". Basically, whoever plays Piter is gonna have to be one creepy dude, one you would never trust to turn your back on. Few do creepy so well as James Frain.
The last role I consider necessary to choose an actor for is the semi-villainous Bene Gesserit Reverend Mother Gaius Helen Mohaim. Yes, there are other major characters, but I'm gonna let them remain uncast because they could end up either cut or minimized to walk-on roles. The Reverend Mother's role cannot be done that way, however; pretty critical character, here. Really, any older actress with gravitas could play this character, but after seeing the most recent Robin Hood movie, I'm choosing Eileen Atkins, who looks amazing for her age and is a terrific actress.