Having a character do something, and for the reader to recognise that it's something they do is an easy way to make characters real. The first time I did it was by accident. I wrote about Isabel, the main character in Adultery for Beginners, trying on clothes to go to a job interview. She's not very confident and at one point she tries on a skirt:
"She sucked her tummy in as she pulled the zip up and looked in the mirror, arching round to check her rear view. No excessive bulges, although her legs looked ridiculous protruding from the bottom hem, two inches of solid white flesh then black ankle socks. Her feet looked enormous, and strangely flat."
At the time I was still doing my MA and one of my fellow students commented, 'I didn't know anyone else felt like that, that's just how I think mine look.' I was pleased, but didn't think much more of it. Now I realise that it was a clue to how to write believable characters: have them think real stuff, that real people think.
It's the basis of a lot of comedy, that feeling of recognition - I once saw a stand up do twenty minutes of hilarious routine based entirely on people's body language at check out queues. It was funny because, well, we all recognised ourselves and our little wiles and secret thoughts and unspoken etiquette. Your characters may be imaginary people, but give them real thoughts and they will become real.