When you start a new story you have to create characters, major ones: the good and the bad, and minor ones again, good and bad. As you write, there will be background characters that are there one minute and then they are gone--sometimes never to return lets call those the extras.
So today we are talking about the majors and a little about minors.
Your characters need a few things to be well rounded:
2- 3 Dimensional
4- Do what they need to do
The majors will generally have all four elements and your minors will have any where from one to three of them. Since they are not the spotlighted character though, you don't need to spend as much time developing them in the story--usually. :) The extras need to say or do whatever and leave...no need to really develop them beyond that.
So #1: Believable. You need to make sure that you don't allow your character to overstep his or her bounds. If she is in a wheelchair she probably isn't going to jump up out of nowhere and fight off would be attackers with her mad ninja skills, round house kick and everything.
#2: 3 Dimensional. As people we want to be well rounded, so should your characters. Think about how they would react in different situations. Do they change their goals often? How do they respond when something doesn't work out? But a key here is be consistent with your character.
#3: Memorable. Tie your reader to your character. You can make him or her identifiable with characteristic tags. Hermione Granger: frizzy hair, super smart, likes to read, level headed, all about the facts. Using things about your character helps people remember them even after the story is over because they are relate able.
#4: Do what they need to do. Give your characters the skills or opportunity to gain the skills needed to accomplish their goals. If they have to get to the Castle to kiss the princess but there's a dragon guarding it, a small bottle of water isn't going to cut it to defet that dragon. Give him time to gain courage, strength and sword skills to win and a breath mint to kiss the girl, okay that last part is more for the girl, but it could help his confidence too.
I often find that if I am stuck in my writing that I have a character (or four) that is missing a part of their development and once I go back and fix it the story can resume smoothly--if writing a story ever goes smoothly. :)