Senses and Sensibility
Nearly all our senses react to strong stimuli like someone threatening us with a weapon. If the protagonist has sweaty hands, a dry mouth, a rapid heartbeat or chills up and down his spine, the reader recognizes these feelings from his own moments of fear.
This works fine in a point of view character, but what if the POV character isn't the one we want to portray as afraid? Then we add the visible cues of fear. We might see the frightened person tremble or widen their eyes. We could hear them gasp or scream or maybe their voice rises to a higher pitch. If we are holding one of their hands, we'd feel it trembling. Some say they smell fear, although I think that's figurative.
Using these techniques and building them to a fevered pitch can create wonderful tension and panic for your reader. And in our line of work, that's what it's all about. When a reader or reviewer says they were terrified during part of my story, I know I've done my job.