General Fear:
* Scared stiff: Extremely frightened.
* Petrified with fear: So scared that you are unable to move.
* Frozen with fear: Similar to "petrified with fear".
* Fearful for one's life: Worried about the possibility of dying.
Apprehension or Nervousness:
* Have butterflies in one's stomach: To feel nervous or anxious, usually before an important event.
* On edge: Feeling anxious or nervous.
* To be afraid of one's own shadow: To be easily scared or paranoid.
Specific Fear:
* To be afraid of heights: To experience fear when looking down from a high place.
* To be afraid of the dark: To experience fear when it is dark.
* To be afraid of commitment: To avoid making promises or getting into relationships.
The best idiom to use will depend on the specific context and the level of fear you want to convey.