As I mentioned in my last post, it's a challenge to write mysteries; you have to get in the heads of so many different people. Today we're going to think like a suspect. What motivates them? How are their emotions hampering or helping the investigation? How can their perceptions (right or wrong) influence your sleuth's case....and your book?
Some people are suspects merely because of their closeness to the victim. Remember that husbands are always the first to be suspected when their wives are the victims of foul play. And when children disappear, frequently the parents are among the list of suspects. In these scenarios, the suspects are likely to be grieving, hostile, and defensive. They may be shocked at the death of their loved one and furious that the police would even consider them as a suspect. These characters may mislead the police out of maliciousness or a desire to implicate someone else and draw attention away from themselves. Or they may plead with the police to help find "the real killer." They're more likely to have many conflicting emotions.
Some people are suspects because of motive, means, and opportunity. Maybe they were at the wrong place at the wrong time. Or maybe they had a grievance with the victim that the police become aware of. They may have secrets that they don't want anyone to find out about--they lie about their whereabouts to conceal an affair. Or they lie to the investigators to make their involvement with the victim seem less significant or incriminating. They can function to send police and sleuths in wrong directions after red herrings. Their motivation is clearly to protect themselves at all costs.
For some, becoming a suspect in a murder investigation is terrifying. Living through their worst nightmare might make some of them exhibit characteristics of guilt---rapid swallowing, shallow breathing, nervous sweating. The investigator could assume guilt where none exists...another possible red herring, this one with non-verbal cues.
Some suspects may even be desperate enough to try to solve the crime themselves---this is the way some amateur sleuths realistically become involved in the case.
Agatha Christie enjoyed using the concept of "the unreliable witness" in many of her books. She created characters that acted odd, lied outright, seemed vacuous, vapid, and ignorant. Then she'd have one of them drop a clue. It would be overlooked or ignored. It might be just an observation that the oddball character made about another character, or something inconsistent that happened that they remarked on. But it would be the truth and help lead the sleuth to their solution.
Or, conversely, you may have a suspect that seems extremely reliable. You've got a doctor, academic, a leader in the community. They make an intelligent observation about another suspect. The investigator goes off on the tangent. But they've sent them off on a red herring. It's up to the investigator to determine if they were sent on the red herring to divert attention from the "reliable" suspect. Or whether it was just poor intuition on the part of that character.
Different characters may have different reactions to being placed under suspicion. By putting yourself in their place, you can enrich your story and provide more realistic characters. They may lie, sweat, fume, but they'll provide color for your manuscript.