Creating believable characters

In my WIP, I switch between two characters. It's challenging to make sure that each character sounds different. I want the reader to hear each of their voices, to be able to tell who they are not only through their physical details but also through their mannerisms. I want the characters to be believable. I also want them to be memorable.  


How do you make memorable characters? I've been thinking a lot about this lately. One way to make them memorable is through physical details. Take Harry Potter, for instance. When we think of Harry, we think of his scar and black glasses. When we think of Princess Leia, we think of buns covering her ears and a long, white robe. 


Also, we make them memorable through their mannerisms. Remember Violet Beauregarde, the girl who was forever chewing bubblegum in "Charlie and the Chocolate Factory?" Violet was memorable.  


In one of my middle grade novels, a character is forever wiping his nose on his shirt sleeve. Gross, I know. But his shirts always look like a slug has slithered across them, leaving a slimy trail behind. It's something that sets him apart from the other characters in the book. It's also something he's teased about. 


So think about your characters. How do they sound? Do they speak in short, choppy sentences or do they go on and on? What are their physical details? Their mannerisms? Have you made them memorable? Distinct? 

If you were to describe your character in five words, what would they be? Please share in comments.