Agree with Clara that this would be SUPER rare in an MBB interview. I wouldn't spend time on preparing this kind of quetions. Spend the time on preparing case and the proper FIT questions instead.
That said, if you indeed encounter such question, think for 1-2 seconds before you jump into the answer - "can I easily support my answer"? Using this particular question, it is easier to give example of being spontaneous, as you can cite a time when you did a last min travel etc. But harder to show you are humorous as that's a lot more subjective.
Also be mindful of whether you answer aligns with what is expected out of a consultant. Spontaneous can be lovely for personal life, but you surely don't want to come across as someone who has no plan, or someone who might surprise your client or parnter in unpleasant ways. So if the answer if yes, suggest to distinguish between how you behave in personal life vs profession life.
Hope it helps,