Is it the description of long blonde hair? Pursed lips? Delicate eyes? Smooth glowing skin?
Well actually those are just descriptions, and while they help to create a mental picture of a character, they do not make the character real.
Is it the fact they get angry? Sad? Frustrated? Or they fall in love? Or they hurt themselves?
Those are feelings, reactions to situations. Like descriptions they are important, and they give a certain amount of emotional definition. But again, they do not make a character real.
Actually the most important thing a character can do is make decisions.
Yes, decisions, it's as simple as that, decisions make a character real. When a character makes a decision then he or she will change from being a passive force within the story to an active force. In fact each decision they make becomes pivotal to them, the plot and all the other characters. A passive character reacts to the plot in order to survive and that is all. Basically they are the novel equivalent of a wet blanket. Why would the reader care about them when they don't care enough about themselves to shape their own destiny?
A character that makes decisions basically says, 'No! I will not just do as I am told, I will decide for myself.' With each decision the reader gets more interested because as a rule they will always be on the side of those who fight for themselves. The decision making process creates conflict, creates drama, and makes the reader ask the question: "What is that character going to do now?" Once the reader asks that question, then the character has become real, and the author has succeeded.
Brilliant Gary! A conundrum we have all sweated and suffered sleepless nights over, and you simplify it to the need for our characters to make decisions!