(someone) thinks on (his or her) feet
"Thinking on your feet" means making decisions about things as they happen. It means figuring out what to do when you don't have a clear plan.
You can talk about someone thinking "quickly" on their feet, or "well":
We're looking for someone who can think well on their feet and make good decisions.
You've got to think quickly on your feet in this job.
"Thinking on your feet" is almost always a positive quality. If you want to talk negatively about someone who makes decisions without planning, you can say this:
He doesn't know what he's doing. He's just making it up as he goes along.