a corps perdu
aback
abruptly
all at once
all of a sudden
bang
by surprise
dash
feverishly
fleetingly
forthwith
hastily
head over heels
headfirst
headforemost
headlong
heels over head
hurriedly
impetuously
implausibly
improbably
impulsively
in a trice
instantaneously
instantly
like a flash
like a thunderbolt
momentarily
of a sudden
on short notice
plop
plump
plunk
pop
precipitantly
precipitately
precipitously
presto
quickly
rapidly
rashly
recklessly
sharp
short
slap
slap-bang
smack
sudden
unsuddenly