How to say "Making a quick decision on the spot" in English

Sometimes you get put on the spot and have to decide before you can even blink. No time for weighing options.

think on your feet

idiom

This nails the feeling: reacting fast, under pressure, and often with a bit of cleverness.

A: How did you handle that question?

B: Honestly, I just had to think on my feet.

make a snap decision neutral

when it’s extra fast

wing it informal

when you improvise, less formal

Skip: "make an instantaneous decision" — Sounds like a robot wrote your script. No one talks this way.