Do your characters lead their own lives?
Is your star in their life’s prime
Or going through a period of strife?
Do they develop and change?
Do they have an emotional range?
The quickest way to kill a novel, script or short story is to populate it with flat characters. Flat as in no depth and in having “bland” reactions to the events depicted in the story. It sounds horrid but readers want to “see” the characters experience pain, laughter, learn from their mistakes, make mistakes to learn from in the first place and so on.
Can we identify with your characters’ plight?
Do they cave in to evil or put up a fight?
Do they have their virtues and flaws
The way we all do and do they have a just cause?
If your characters don’t seem real to you, they won’t do so to anyone else either. You’ve got to like, love, hate or feel something for your creations. How else can you write convincingly about them? Draw on your own experiences of your reactions to stressful events to be able to portray those in your fiction doing much the same thing. Without overdoing it and taking care the information isn’t given to the reader all at once, the more mundane things you get your characters doing in between the “exciting” bits will make them seem more real.