Too often writers sidetrack a scene with descriptions of churning stomachs and beating hearts and swelling throats. If a monster leaps from a closet, shouldn't the focus be on the fearsomeness of the monster—its dripping fangs and razor-sharp claws—rather than on the rapidly beating heart of the character the monster scares?
Writers sometimes believe that to play fear they need to show their character being afraid, and this leads to the usual clichés about sweating brows and somersaulting stomachs. These distract the reader by taking the focus off the object causing the emotional response, and reducing the character's response to a purely biological reaction.
Instead, it's much more effective to show what makes the characters afraid, in such vivid and convincing detail that it terrifies not just the characters, but most of all the reader. It's easy to write that a character is afraid, so easy that anyone can do it. But if you can make the reader feel afraid, or angry, or tearful, then you'll have a reader for life.