Each character in our story should be designed so they are memorable as possible. When I think of the multilayered elements that make up a unique and memorable character, the sound of their voice in dialogue always stands up for me to take notice. Gollum from Lord of the Rings, Hagrid from Harry Potter, Emperor Palapatine from Star Wars: Each of these characters had interesting background stories and physical traits, but what really brings them to life is the unusual way they spoke, a combination of motivation, dialect, and vocabulary.
So, how do we bring that voice out onto the page and into each line of dialogue (and even their thoughts, if we write from their POV)?
One technique that could help get you into the head of a character is to write strictly from their POV. Not as part of the story itself, but as a separate exercise. By this I mean, take a black sheet of paper, get into the mindset of that character, and then start writing. Don’t stop until the page is full. Write a letter to the character’s mother; write an editorial essay as if they were in school; scribble a journal entry that fills a page with the character’s words. Write whatever they are thinking and feeling in their own voice. Don’t stop to ponder their words, just let the characters thoughts become your own and guide your pen.
If you have characters that sound similar, or even if you want to refine the more unique sound of your antagonist, give this technique a try and see what comes of it. You might be surprised.