In my current novel, I'm dealing with very scary creatures that never fail to fill my protagonist with fear, but I always have her doing the same thing. My character always seems to simply widen her eyes, but what are some other common human reactions to fear that I can use to add some variety? I'm not talking about fight-or-flight, just expressing human reactions to fear/being in scary situations like screaming and such!
As an author, your job is not so much to show that your character is afraid, but to make the reader afraid for them. The physical expressions of fear are far more often played for comic effect. (Think about how they are used in the movies. Almost any time you see a character's face expressing extreme fear, it is for comic effect.) If the reader feels fear for the character because of the situation they are in, and because they care about them, then the reader will feel the fear themselves and will project it onto the character without you needing to show it. Again, think about how this is done in horror movies. It is all in the lead up: the cheerleader innocently walks down the dark corridor where the audience knows the ax murderer is hiding. It is all in the pacing, in the buildup. A sudden sound or movement is then all it takes to get the audience to jump out of their seats. Don't describe fear; create it.
The primary fear reaction in humans is not a facial expression. It is fight or flight. Focus on what the character does in the face of the monster, not on what they look like.
Fear in Wikipedia states in "Signs and symptoms"
Many physiological changes in the body are associated with fear, summarized as the fight-or-flight response. An inborn response for coping with danger, it works by accelerating the breathing rate (hyperventilation), heart rate, constriction of the peripheral blood vessels leading to blushing and vasodilation of the central vessels (pooling), increasing muscle tension including the muscles attached to each hair follicle to contract and causing "goose bumps", or more clinically, piloerection (making a cold person warmer or a frightened animal look more impressive), sweating, increased blood glucose (hyperglycemia), increased serum calcium, increase in white blood cells called neutrophilic leukocytes, alertness leading to sleep disturbance and "butterflies in the stomach" (dyspepsia). This primitive mechanism may help an organism survive by either running away or fighting the danger. With the series of physiological changes, the consciousness realizes an emotion of fear.