Anytime I write, I always build emotions into the characters. If I don't, they're not believable and I end up with a stack of cardboard, like so:
Cardboard has no feelings. Cardboard doesn't interact with its environment or other pieces of cardboard. In fact, aside from serving a very basic function, cardboard is not all that useful in day to day life. Characters who don't have emotional responses to their environment are the same.
Get Into the Character's Head
Understanding a character is pivotal to understanding how he or she will react in certain circumstances. To get to that point, I developed an extensive backstory for my main characters in my novel Legacy. Each had their own quirks and habits, all of which were influenced by things that had happened to them in the past.
We're the same way when you think about it. As a child, if you're conditioned to receive praise when performing well in school, chances are good you'll become an adult who is highly driven, always seeking that next big accomplishment. Conversely, if there's no value placed on education as a child, it's possible you won't pursue higher education as an adult (not that it's impossible, mind you, just not quite as likely).
For me to get into my character's head, I had to think about how different events in their backstory affected them and how that would play out in their lives as they grew up. One character, named Matt, received accolades from his father based on his performance in sports, which drove him to pursue football in his college years. Unfortunately for him, there wasn't any value placed on education, so he wasn't diligent about his studies and eventually flunked out of college. This caused him to dive into a downward spiral because he no longer received praise from his father and, subsequently, he had an identity crisis. About half of that entire backstory never made it into my novel, but it did help me to get into the character's head and think about how it would affect him and how he'd respond to life in general.
Act it Out
Close your windows before you do this so your neighbors won't judge you. Sometimes when I write, I get stuck on how a character might respond in a given situation, even with extensive backstory. It happens. What I found to be helpful was to act it out (sometimes with dialogue if I was alone) so I could try to feel the emotions for myself.
The key to that process is to identify the emotion itself. Until a label is attached to an emotion, it is a difficult thing to address, much like in real life. You can't deal with what you don't define.