At its most basic FIML is a way to ask your partner what they are or were just thinking, feeling, perceiving, or meaning when they said or did something that communicated something to you. And then it is a way to get a good answer from them, an answer that completely satisfies you both.
FIML works with data that is as immediate as possible. It works with our “working memories,” the stuff we actually have in our minds as we speak and listen (not the stuff we can call up quickly from memory but that is not actually there during the speech event.)
People often speak more vaguely than they listen. Listening often is more precise in its details than what the speaker was saying. Listening focuses on less of the discourse, sometimes more clearly.
The act of speaking takes up space in working memory and thus can have an atmospheric feel; during speaking the working memory is robustly occupied with putting out words. Listening can be more sensitive, having sharp moments disturbed by confusion. An unknown group of people coming up an apartment stairwell is an example of a typical act of listening.