Making your characters real is really not much of a mystery, even if you're writing one.
Still, there are things that a writer must remember when placing their main, secondary, and antagonist characters on the page. Here they are in no particular order:
1. Every character needs flaws, especially your main character who must change and grow through the story.
2. Every character must have some redeeming qualities; yes, even the antagonist. Perhaps the antagonist lies through her teeth on every account, but keeps a beautifully kept yard or has an eye for formal wear.
3. Every character needs to have characteristics specific to who they are. These characteristics may be body mannerisms, dress style, the way he drives his car or the way she avoids speaking in public.
4. Every character needs to stay in character. If it helps for you to make a list, do it. If it helps you to remember hair color, eye color, or what your main character's name is, write it down or use a picture from a magazine to keep you on task.
5. Every character needs to be as real as your next door neighbor. That means you may know more about your main character than you will use in your book. But you will know the insides and outs of your main character in case something comes up and you need to know her favorite type of music, for example.
I tend to mix up character names in my books, so it's a good idea for me to write them down and have them handy. You may forget the particular flaws given to a particular character or the redeeming qualities, or even the particular body mannerisms your protagonist has.
If so, make a list and have that list handy.
You won't regret it.