Breathe life into your characters and make them 3-dimensional

We often have difficulties to connect with a character; we can't put our fingers on it, but something is missing. In many cases it's the description or interaction with others. Real people have many sides to them, they are happy, they are sad, they are grumpy or even angry at times. Think about your friends; they have traits we love and traits you probably could do without, but all together, they make them complete - real.
Like the Rubik Cube to the left, we see them from different angles. The goal is to get them walking and talking like in a movie. Describing scenes is important, but so are our characters.

Looks are only one thing and I've already explained how to do it best here. But this is only one dimension. What about their behaviour? Is your MC a calm person, polite, reserved, moody, loud, withdrawn, laughs a lot, cries easily? You get the picture. It's important and very handy to have a good think about how your character should be and stay, if possible. Consistency is key, unless they go off the rails and begin to change.
The reason being is that especially in writing, you need the reader to sympathise with your character - good or bad - so every move is understandable and acceptable. The second your reader thinks: why would s/he do that? That's absolutely stupid! you've dragged your reader out of the story and most likely lost him or her.

I use my character's thoughts to get across their reasons for doing things, or I let them mutter, I let them call their friends and either rant or seek advice. I let them meet and judge each other.

For example. Celia is a very good friend to Emily, Tom witnesses it and really appreciates the trait of hers.

It's also good let them have a habit, like blowing a lock out of the face, biting fingernails, running fingers through the hair, rocking back and forth, wrinkling the forehead a specific way, make funny noises when nervous or excited. Those are all things that are noticed by others and makes the character more alive.
Let them interact differently with your other characters. You won't be the same with your boss as you would be with your best mate. Let them have a particular phrase they use and let others react to it.
Has the person a pet? A love for being colour co-ordinated, is the person a neat-freak or messy, there's so much to play around with. Just look at your friends and see how different they are, that's a great way to start with building real characteres that grab your hand and tow you with them through the book.