Science Fiction

The source material has limitations. It is difficult to make characters have depth when in the novel they have none. If you sit down and try to describe the characters of Dune you can really only do it by their job or what they did in the novel. You can't describe their personalities. Compare this to Star Wars.

But, the book characters were intentionally written to be pretty emotionally flat? The Gom Jabbar scene... Jessica showing emotion doesn't make it a bad scene, but it kind of undercuts how the Bene Gesserit work. Their whole thing is conquering their emotions and being composed and in control all the time. Jessica's turmoil is internal while her face is stoic. That's her whole character at that point in the book. she's not a very good Bene Gesserit, but she's faked it real well.

Except Duncan and Gurney. They should have had personality. That's Their purpose in the books. To be the ones who show Paul what being a real human is like beyond the Duke (laden with responsibility and the knowledge that his entire house and the thousands of people that rely on it are teetering on a knife's edge) and Jessica (basically a magic robot concubine who was raised from birth with the sole purpose of furthering a generations long genetic project her ~~captors~~ teachers were working toward). They're meant to be a breath of fresh air that give Paul the foundation to be a real boy.