How does startle response work? Why are some people more easily startled than others? What might cause someone to not startle at all?

Startle response, or flinch, is really a deep subject; it's part biology, part psychology. The psychological aspect is easier to understand, so I will begin there.

People startle to things they perceive as a threat. An example would be if you tried to pet a turtle and it were to snap at you; if it bites you, you will have less fingers. In the same way, people flinch at things they do not expect. If you were to go noodling, and feel something hard and turtle-like, you would likely flinch because while you do not know a turtle is there, there is still a perceived threat. People who startle less easily may just perceive no threat, be it by conditioning or a prior experience.

Biologically, the startle reflex is caused by various things. First and foremost, the intensity of the stimulus is responsible for a startle. In auditory startling, for example, hearing a loud sound triggers a reflex in the ear, reducing the decibels heard. This reflex does not begin until the volume approaches 80 decibels. The reflex usually starts in the brainstem and is transmitted down to the limbs where the reflex is acted out. Someone who is not easily startled may have something inhibiting those neurons. It could be genetic or chemical, or possibly both.