First, it's important to understand that we, as human beings, have involuntary responses to certain situations and stimuli. These are visceral responses, things we really can't control. In high-emotion situations, our hearts beat faster, mouths get dry, hands sweat, etc. These responses are tied to the body, not our conscious thought-processes.
When writing, especially in scenes with emotional impact, we should draw on these responses and make the reader feel them as well. We've all heard, "show, don't tell" and we probably know better than to say, "She was scared." Instead, we might say, "Her stomach clenched with fear." But is that enough?