One thing that can help is to "borrow" personality traits from people you know, and give them to characters in your work. I'm not suggesting that you should take a person you know and just make him or her a character, but rather: watch people as they interact; does one person consistently take everything that is said to them as an attack? Character trait. Is one person a problem-fixer, while another prefers to talk endlessly but never
do anything? Character traits. Know anyone with a phobia that somehow could relate to your plot? See how that person reacts when dealing with that phobia.
Things like this add depth to your characters. For major characters in a novel-length work, some significant depth is probably necessary, or readers will get bored. For minor characters, they need to have at least a little depth, or they'll become interchangeable, throw-away characters, and at that point you might as well combine them (consider Rosencrantz and Guildenstern from
Hamlet; they were so interchangeable that Shakespeare even referred to that fact a couple of times in the play.)
Part of the reason for putting a couple of quirks into a character is that it allows the reader to identify with that character, which does a lot of your work for you; if one of your characters reminds a reader of someone they know, then they'll unconsciously fill in details for that character, using their own friend as reference, and that will allow the character to become richer in their minds without you having to explain every little detail. And one mantra that you will hear over and over is "Show, don't tell." If your character John is a jerk,
show that through having him
be a jerk, rather than stating (outright in the narrative, or through dialog) that he's a jerk. In fact, you can use this game to reveal insight into
other characters: if John's been unfailingly a nice guy, but Jane tells Ann that John's a jerk, then you have given the reader something to chew on about Jane, and possibly the interrelationship between Jane and John, and (depending upon how Ann reacts) you can extend this to hint at the Jane/Ann and Ann/John relationships as well.
Avoid putting a character in the book that is
you; too often, because of how strongly a writer will identify with such a character, it will become overbalanced. Since the writer knows everything that's going on, it's natural for the
character to have an unrealistically-canny insight into what's going on. (This is related to the
Mary Sue phenomenon).
There is a pretty good forum for writers to share ideas and discuss things like this, as well as to post their own work and critique the work of others, at
Hatrack River.
-BP