So I have a question about writing a mystery/crime novel. I've read that most famous crime novelists, like Agatha Christie, would write a rough sketch of the story first and then, depending on which character aroused the least suspicion, connect him/her to the crime. This was determined by picking a character, who, for instance, seemed to have little or no relevance to the crime. The narrators/MCs would keep talking about all those other people so convincingly, our mind wouldn't go to those side characters. In essence, the mystery and suspense was ensured because the reader would have little exposure to the character. While I don't dispute the technique, I am wondering if it is possible to retain mystery/suspense even if the suspect is an integral character who, say, the MC is infatuated with. Sort of like someone so close to you (as a reader) that your mind doesn't go there. My own novel has that sort of angle, and though I have taken some necessary precautions, I'm not sure how true and tried this technique is. There could also be technical reasons for why the person is not a formidable suspect, but despite the MC's focus on the character, is it possible to keep the reader in the dark long enough for it to be a surprise? If so, do you have examples from literature of when this was successfully pulled off? BTW, this is in context of first person POV.