Think on one's feet

"There will be wine," said Medora. "Drink it. There may be toasts. Be ready to respond." Abner could think on his feet—speech would not fail.

The political canvass had given freedom to his wings; he had learned to think on his feet, to meet interruption, to parry in debate.

Clemens had to think on his feet, adjust on the run, make split-second decisions and, if he messed up, contend with the consequences.