Answer:
Personally, I believe you have to look into that character's personality and figure out their speech patterns. Do they have an accent? Do they have any habits, like talking with their hands? It's good to be descriptive. Look at where they come from as well. Are they a fancy and dignified character who had a higher education? They might speak more complex and with complicated words, or if they're wise in their thinking, they may slow their speech to give themselves time to think before they speak. Their mental state is something else to consider. If you're writing a villain who's off-their-rocker, their thoughts might be a bit scrambled and so will their speech. It would be quick-paced and unorganized.
Hopefully this was helpful! Just let me know if you have other questions and I'll try my best to help.