The core idea behind Gumshoe as a system is that 'Core' clues are successfully found with an appropriate use of an investigative skill.
There are a whole bunch of interpersonal investigative skills such as Flattery, Intimidation, Reassurance. Can an inappropriate use of one of these skills 'fail' or make a situation worse, or would the investigator instinctively know that now is not the right time to use Flattery for example?
As an example, lets say we have a witness to a crime who is petrified. They have a Core Clue that can be retrieved by the investigators. However, I would like to portray this guy as someone who hates to be intimidated and would rather fight back than be cowed by any such attempts.
In this circumstance what do I do if an investigator tries to intimidate them? Do I let them fail, making things worse (as long as the core clue is still obtainable some other way of course), or do I assume that their skill in Intimidation tells them this would not be a good time to use it?