Thread: on my feet ¿? CITY OF GLASS

X“Debo aprender a pensar rápido y con los pies en la tierra”.X

"Debo aprender a tener más agilidad mental".

“Tendré que aprender a ser más asertivo”.

La que te guste, pero no la primera, como bien me corrigió Mariaklec.

We call it “agilidad mental” to the ability of giving immediate and correct response when being involved in an argument, or being responsible to solve every doubt after a lecture, or in an extremely difficult situation where there’s no time to think as MariaKlec example with airplane, when dealing with aggressive audiences, etc. It refers to a good problem solver.
ASERTIVO, (from aserto) may partially work since it only makes reference to identification of the correct answers. You are an “asertivo” when you know how to distinguish the correct answer. In this case for instance MariaKlec has been quite “asertiva” in giving us a hint.
And my “agilidad mental” failed to give a more accurate answer.
Note: Acertivo vs. Asertivo
Acertivo (from acierto) ACERTIVO would be the most accurate term to be used in your sentence, it refers to all of the above. But technically speaking the word does not exist. It’s not listed in the dictionaries. We use it quite frequently even though almost every one confuse it with aSertivo.
Asertivo (from aserto) is a person capable of distinguishing good from wrong, yet it does not necessarily makes him a good problem solver.
So I would definitively go for “agilidad mental”.
Astucia or Sagacidad may not work in this case.
Astucia involves a trick or a lay to be granted as an answer, or to avoid as a response.
Sagacidad refers more to giving the correct steps before a given situation.

Thinking on your feet means to solve problems as they come up while you're doing something. For example, a lecturer has to think on his/her feet to answer questions from the audience well. The lecturer must come up with a good answer on the spot---no time for research or to think things over. Another example would be an airplane pilot who has to make moment to moment quick decisions---no time to look things up in a reference manual or to consider a list of options at length. Another example would be a person interviewing for a job---to get the job you would have to think on your feet during the interview.

I can't quite get the context in the original quote (something is not quite right with "unspected call"---maybe an unexpected caller?), but maybe if he had been thinking on his feet he would have realized during the phone call that the caller could have been useful in solving the case. If he had been thinking on his feet he could have asked the caller questions that would have helped him solve the case.