1) The character must be a character.
They can't be flat or shapeless. They can't be neutral or wussy. They have to be for or against something. That's important. You have to give your reader something to react to. You can have them hate or love the character, but the reader has to have a reaction.
2) The first time he appears, the character should perform an act that characterizes him.
This goes back to Sunday school: don't listen to what people say, watch what they do. Your reader will watch what the character does, and it should be something important. Is he a thief? Have him steal something. Is he honest? Have him give back a nickel to make correct change. Generous and sweet? Have him over-tip the waitress after charming her.
And make that waitress important later.
If he's not generous and sweet, have him charm the waitress and then stiff her.
That doesn't mean to make the charecters flat. You can add contrast later, and should. Just show what they essentially are, in their core, first. More about adding contrast in these posts:
3) The characterizing act must be both pertinent and characteristic.
That means you should show the most important characteristic of your character, not a side characteristic. If he's supposed to be courageous, don't show first how kind he is, etc.
Thanks for reading!
Here’s two interesting 99c short stories for you to read: (More fiction coming soon.)Nag Is Hindi for Cobra (All Formats)