Writers hear all the time that our books need more emotion. That this character needs to have a bigger emotional impact or that scene lacks believable emotion. So... how do we write emotion?
The easiest way is to tell us, "George was angry." But that doesn't make it believable or have much of an impact.
The next easiest thing: show us using basic gestures. Angela Ackerman posted an emotional thesaurus, listing appropriate or likely responses to certain emotions on The Bookshelf Muse.
If you're looking for something a little more complex, I've already linked to The 5 Stages of Grief on Show Some Character. Take another look if you're stuck. :) It doesn't just help with depressed characters. The 5 stages of grief are applicable to any character experiencing any level of distress. (And your characters better be experiencing some sort of distress in one form or another.)
From the same website: some advice on choosing which emotion to show. Here's Dramatic Frustration: Remember to Keep the Emotions Real.
We're not our characters and we don't share the exact same experiences as our characters. But we can use our experiences and emotions to make things just as scary (or scarier) for them. In I'm Not Them, But I'm Just As Scared, Lili St. Crow shares how she fuels emotion using her own past. Good stuff. :)