Synonyms - "immediately"

abruptly, all at once, bang, by surprise, forthwith, hastily, head over heels, headfirst, headforemost, 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, quickly, rapidly, rashly, recklessly, speedily, startlingly, sudden, surprisingly, swiftly, unanticipatedly, unaware, unawares, unexpectedly, unforeseeably, unpredictably, without notice, without warning

 

Warning: You cannot just replace "immediately" with any of the words above. They need to be used in context. It depends on where, when and how you are using them. For example, you cannot use "head over heels" in front:

E.g. Suddenly, the man took out a knife.
E.g. Head over heels, the man took out a knife. (X) - This is incorrect.

To be safe, use only adverbs (+ ly):
Abruptly
Impetuously
Implausibly
Impulsively
Instantaneously (spelling)
Momentarily
Surprisingly
Swiftly
Startlingly
Unforeseeably
Unpredictably

Or:
All of the sudden
Without warning 
Like a flash
Like a thunderbolt