Suppose someone reading Anne of Green Gables asks a question about what happened before the story begins. For example, what did Anne have for lunch 37 days before her arrival in Avonlea?
It is easy to see that this question does not have one true answer. There is no such thing as what she really had for lunch, because it is a story, and that meal is not included in it. On the other hand, despite the lack of any absolute truth here, some answers remain more reasonable than others. For example, “She had salad,” is a more sensible answer than “she ate crushed glass that day.” Just as I said in regard to “why” something is the case, one can give a partial answer, in the sense of showing that some options are more intelligible than others, without being able to exclude some options entirely.
These same things will apply to questions about a work of historical fiction, although the intended historical context will provide additional ways to show that some answers are more sensible than others. Thus if a story is set in ancient Rome, the claim that someone had corn for lunch is unreasonable due to the historical context, although not as unreasonable as some other possibilities that you could suggest.
Now consider a counterfactual question about your current situation: “What would you do if it were 120 degrees Fahrenheit in your house?”
There is no fundamental difference between this and the case of historical fiction. In effect, we just created a story about you: “It was 120 degrees in your house. You…”
Like the case of historical fiction, some answers will be more sensible than others, but there is no thing that you really would do in that situation. The story didn’t really take place, but if it did, it would have taken place with a lot more concrete detail, and that concrete detail could determine the specific answer to the question. If Anne of Green Gables were a true story, her concrete situation would have determined what she had for lunch that day. And if it were really 120 degrees in your house, what you would do would depend on how and why things got that way, as well as other factors in your concrete situation.
Some philosophers have spent a lot of time on this kind of counterfactual question, apparently largely from a desire for absolute answers. For example, some suggest that a counterfactual is true if the claim is true in the nearest possible world where the antecedent is true. In a similar way, Molinists argue that in order to be omniscient, God has to know what you would do if it were 120 degrees in your house, and that it must be one specific thing, so that there is one thing that you really would do in that situation. They call this kind of knowledge “middle” knowledge, namely something in between knowledge of what actually is and knowledge of what merely might have been.
All accounts of this kind are wasted effort. The brief account above is sufficient.