This is so much easier said than done, but I wrote the dialogue between my characters as naturally as I could. I thought:
“How do my friends and I interact and speak to one another?” This is how my characters who are familiar with one another interact. Just how I would with my friends or my significant other. I think about what I would say to my friends or how my friends respond to me and vice verse.
“How do I speak around someone with more authority than me?” This is how I have my characters speak around the “General” in my story. He’s the antagonist and they have to walk on egg shells with him. They treat him with respect, so the dialogue is more like how I speak to my boss or a teacher.
“How do I speak to someone I’ve just met?” This is where I cut out cussing and the characters talk more politely and more formally.
“How would I speak to someone I don’t like?” This is where they might exchange insults or have more vulgar language again. Or even some sarcasm.
You kind of have to put yourself in that character’s shoes and consider:
What was said to my character?
Who said this to them and what is the relationship between them?
Do they like each other? Do they not?
How would this character feel or react to what was just said to them by this other character? Physical feeling is also an important reaction by words.
You sort of grow to understand your own character’s emotions and reactions and it starts coming more naturally. Sometimes you just have to begin writing and start developing who these characters are so that you get to know them too. I’ve had to go back to old chapters and rewrite some of the dialogue my characters shared so I could make them interact more like who they are (after I got to understanding them myself).
So for your novel, start thinking about the kind of person Brute is and the kind of people his lackeys are. How would they act with each other behind closed doors and how do they act out in public?
I always find it nice to have characters who are different from one another in some sort of way. Maybe one lackey can be a quiet guy, sort of mysterious. Another can be short-tempered and difficult to work with. One could be Brute’s best friend who is there to listen and support him. Another can be the comic relief who is always happy and cracking jokes. Sarcastic characters can be fun too. Things like that. It sort of helps create different interactions between your main character and the others that helps differentiates them.
And another important one would be Brute’s relationship with Darksider. Is he intimidated by him? Does he do whatever is asked of him? Is Darksider stern or does he treat brute as a son?
Sorry for the length of this, but I’m hoping to help you get started on writing the project to see if you catch a flow with it. It seems like you might do best to try writing some pages on your own and sharing it with someone for editing advice (who can suggest how you might change the dialogue or make it more feel more natural or flow more evenly). It’d be easier to help you with dialogue if there was an example of your writing style.
But you should definitely at least give writing a go even if you’re new to it. You can always grow as a writer and I know people here give great tips and advice on people’s work when they ask for reviews or advice.