One mistake a lot of beginning writers make is to write flat characters. By flat I mean the character is unremarkable, forgettable, cliché, or even interchangeable with other characters. How can we make our charcters feel real?
1. Character Sheets
I write character sheets on all of my named characters, even if they only appear a few times in my stories. A character sheet is a list of everything you know about your character. Do an internet search for "writing character sheet" and you'll come up with lots of examples.
2. You Know Everything. Your Reader Doesn't Have To
By the time you've filled out a page or two about your character, you are pretty familiar with who that person is. A temptation is to add all of the great new information to your story. Don't. The character sheet is to inform you. Just by knowing your character well, you will write them differently.
3. Mix It Up
Okay, you know your character well. Now think of one thing they would never do, or something that is totally the opposite of where they came from and who they are. Now make that part of their character profile. Don't we all have internal conflicts? Aren't we all walking contradictions? Your characters should be too.
Certainly this list isn't exhaustive, but it's a start, and if you do these three things, your characters will feel more real to your readers.