I have never heard of someone being charged with a crime because he discussed a hypothetical crime for a work of fiction.
I guess it's easy enough to imagine two writers discussing plot details for a book who are accidentally overheard by someone who thinks they are plotting a real crime. Frankly though, if the police came by and you explained that you were working on a book, and there was no other evidence that you were planning an actual crime, I think the incident would be promptly dropped. Maybe, possibly, they would put a note in a file somewhere so that if such a crime was actually committed, they'd come back looking for you. The police have enough crimes that have actually occurred that they must investigate that they can't spend much time on so-and-so thinks his neighbor is plotting a crime. Conspiracy tends to be a charge made after the crime is committed, not before. About the only conspiracies that police investigate before a crime is committed are threats against the president and terrorist attacks.
It is not a crime to discuss crime hypothetically. People do it all the time. If a prosecutor tried to charge you with conspiracy, they would need more than just, you were heard in a public place saying, "Suppose someone wanted to kill his wife ..." They'd have to show that you actually bought the poison or hired a hit man or something of the sort. I suppose if your research includes buying bomb-making components to see what they look like and how they fit together, and you assemble a bomb in your garage, and you're overheard asking about Governor Jones travel schedule and security arrangements, and you rent an apartment overlooking the square where the governor plans to give a speech and the police find the place empty except for a chair and a sniper rifle, etc, the explanations might become more difficult.