Embark on your novel at peril if you haven't nailed your characters. Character can be done last but it involves headache inducing rewrites. Instead get 'em nailed first. Number one- get the name right, this goes hand in hand with what I call the moment of squint-truth: this cumbersome phrase refers to the inner mental squint you make to try and capture your character in some defining moment or relationship. For one of my characters it is the fact that he has an unnecessary 'enemy', for another that she has a huge first aid kit- these captured moments seem to set up the right improvising energy needed to make the character thrive. It's rather like getting the right hat for a role on stage. Wear the right hat and you know how to speak, go on stage in the wrong hat and you're tongue tied. The right defining feature will also, usually, highlight a contradiction within the character- people who are internally torn are always more interesting- and lifelike.