As I sit in a bookstore coffee shop I carefully observe the hustle and bustle around me. From the middle-aged woman speaking just a little too loudly about her pending divorce to the young man who sits alone and can't seem to maintain eye contact with anyone in the room – I watch them all.
No, I'm not a serial killer or enemy of the state. I'm a writer researching characters.
One of the most effective ways to ensure your characters 'act' like real people is to always think of them as real people. Why do they feel/think/act the way they do? Would your sister/brother/best friend talk like that?
Keep in mind that characters typically change over the course of a story. They learn life lessons and new perspectives that cause them to grow and change. These characters are referred to as 'dynamic' characters.
Static characters, on the other hand, remain the same throughout the events of the story. They don't share in the changes of heart and mind that move other characters.
Of course, you could have characters who are moved by some events but not others, giving your story a mix of static and dynamic. Whatever qualities you chose to give each character, make sure it is consistent for the character. Consider their background. Culture and gender play great roles in making us who we are. So does financial status.
I find it helps to imagine my characters as new friends. I'm curious to learn about their past, their hopes and dreams, and their present reality. I make character profiles for each one - major player or not - to determine how each will move the story forward. That is the point to developing great characters; that they might bring personal experience and bias to each scene and intrigue the reader further.
When you remember the greatest stories ever read, it's the characters who come to mind. They were likable or vile, funny or dry, impressionable or 'set in their ways.' The characters involved are what make the events of a story important. Flat and uninteresting characters are an injustice to your story. Remember to always keep 'em real.