In the introduction of the author’s definitive edition of Speaker for the Dead, Orson Scott Card said something that is very true that I never really implemented in my own fiction.
I’m sure you’ve noticed that every person has different “voices” they use when talking to different people. They change themselves (even if it’s only slightly) to fit in the interaction in which they take part. Essentially, we each have multiple personas; one for each relationship we have (including strangers).
If you look at the psychology of it, I’d guess that it has to do with our need to fit in. Humans are social by nature. If you are in a group of people, or even just in the company of one person, you want the other party to accept you. Your mannerisms, attitude and the like changes to something that you believe will cause them to include you. Ergo, we have a collection of masks hidden in our psyche, and we swop them continually in order to fit the different situations/contexts we find ourselves in.
If you apply this to characters in a story, it is obvious that for every relationship, you’re going to have to develop a persona for your character. If there are three characters in a story, you’re going to need four personas for every character—one for each relationship. One and two, two and three, one and three and when they’re all together.
While this is probably going to be a lot of hard work (especially if you have a lot of characters), I think it’ll add a depth to your character that you wouldn’t be able to get otherwise. And you’ll have a lot of fun doing it.
On another note, we, as writers, have another mask. It’s the persona we assume when we write things down. The smart people call this voice. It’s sort of like a public mask, the one we put on when we speak to the world. The question is this: Will you craft a porcelain mask, or go out in public with a cardboard face?