I'm not good at them.
All right, I set up their desires to conflict, and as directly as possible, but that's possible while their personalities are just about carbon copies of each other.
And the Greek god technique of characterizing the minor character -- well, I know enough to not chose the Greek god that most resembles my main character, but it's a bit part, really. If the character is more than bit, making them too like the god starts to run into dimensionality problems; if your character can be identified by type, your characterization probably needs some work.
So the gentle art of remembering that your characters have to be constructed and not just reflexively grabbed from whatever seems most natural to you, because that will be the character type most natural to you, and then constructing them, preferably before you write enough of the outline that it would be hard to rip it out and change the character -- which, of course, you need often enough because character is plot -- is absolutely necessary and a real pain.