When you aren’t writing — out and about at work or shopping or with friends; observe how people act, how they dress, and how they talk. Take note of the weather or the way the sky looks at sunset or sunrise. Listen to the various sounds in the world around you. All of this can be incorporated into what you write.
Pay attention to your own feelings in various situations. Write them down in a notebook, along with other observations you make. It doesn’t have to be a journal or a diary, merely notes on emotions, how you felt when you were angry, sad, happy, excited. If you need to write an emotional scene, think about how you felt with that emotion and transfer it to the characters involved.
When you write you’re putting yourself out there for the world to see. Every word, every sentence is a part of you. The characters are drawn from the people you know, have met in passing, or seen at the local coffee shop. As a writer, you live in two worlds — one you create for your characters and the one you live in every day. You cross the line between these worlds each time you put pen to paper, bringing the real world to your created world.