suddenly

to realize or notice something unexpectedly

to experience a quick onset of fear

to become scared in an instant

to be surprised or shocked unexpectedly

to become watchful and attentive unexpectedly

to become cautious or suspicious unexpectedly

to cease an action or movement without warning

to start without warning

to become aware of something unexpectedly

to come into sight unexpectedly

to recall a memory without warning

to vanish from sight unexpectedly

to disappear from sight quickly and mysteriously