The Things I Learned in Fiction Class

Your characters are real people, not stereotypes

Be nice to them. Treat them sympathetically, like real people. This applies to all your characters, even your henchmen. Give them human traits. If your MC is a loudmouthed football coach, make him like poetry. Description always, always helps. Describe your character in detail, from the gleam in his eye when he sees a good fight, to the lackluster birthmark on his chin. This will make him credible.

There's something we discussed in class. It's called the 180 degree shift. You want your character to surprise the reader by exhibiting a trait, an action, that goes against what you would normally imagine that character doing. It can be a small thing (like the football coach enjoying Keats). Or it can be bigger. Maybe that same loudmouthed football coach lost a kid to cancer and is heavily involved in organizing charitable events for Children's Hospital.

My fiction professor had reservations about too large a shift (the full 180). He advised us to stick to things that didn't challenge credibility too much. But if you want to try a radical shift, then go for it. Fiction is all about bending the rules.

Above all, remember that every character has their own motivation. You may not know what that motivation is when you start writing the character, but it should become clear to you as your story proceeds. The man who tried to kill the king is not just Hired Assassin Number 63, who gets his jollies out of killing people with a big knife. Maybe there was a famine on his farm and he has to feed his family, and he doesn't really like his profession, but he needs the money. Or maybe his parents died in war, and he's an idealist who believes that the totalitarian regime will bring peace, and he intends to wipe out the rebel opposition to help the ruler in power, even if it means sacrificing his own life. At least give him some raison d'etre.