1‘she suddenly began to laugh’
immediately, instantaneously, instantly, in an instant, straight away, all of a sudden, at once, all at once, promptly, abruptly, in a trice, swiftly
unexpectedly, without warning, without notice, on the spur of the moment
informal straight off, out of the blue, in a flash, like a shot, before you can say Jack Robinson, before you can say knife, in two shakes, in two shakes of a lamb's tail
We take a look at several popular, though confusing, punctuation marks.