とっさに

とっさに is an adverb meaning "instinctively", "on the spur of the moment": something sudden happens and you react before you have time to think. It goes before the verb, which is usually a quick physical or spoken reaction – dodging, grabbing, blurting something out.

とっさに or すぐに

すぐに is about speed: the action comes soon after. とっさに is about the lack of thought: the action is a reflex to something sudden. A planned action done promptly takes すぐに.

• × 家に帰ったら、とっさに宿題をします → 家に帰ったら、すぐに宿題をします

Usage notes

These go beyond the adverb with a positive verb.

とっさの before a noun

Before a noun it is とっさの: "split-second", "sudden". とっさのことで means "it happened so suddenly that…".

とっさには with a negative

With a negative verb it often takes は: you could not manage it in that instant.