When they talk about the idea of "the least likely person did it", why is such a person the least likely?
(Really, who determines these likelihoods? Isn't the idea of quantifying such a thing entirely subjective?)
What I've come to realize is that the characters designated as "least likely" are often considered "least likely" because they are present not as individuals, but as functionaries. They're involved in the story -- seemingly -- not due to their personal lives, but due to their work.
I'm revisiting one novel where the culprit turns out to be the relatable-young-protag's boss, who seemed to have no personal connection to anything in the story. Honestly, I find this one a bit thin.
There are a few others where the reader gets to know, and, presumably/hopefully, love some family of characters in which there has been a murder, to the point where the reader doesn't want any of them to be guilty. When it turns out that the murderer is the doctor, or the maid, the reader is supposed to feel comforted that it's not one of the people they liked... not one of us. I personally hate those sorts of endings.
The "They meant to murder me!" is a different type of least-likely person, but I feel like once you've read one with that twist (raise your hand if one is in your top five Christie books), you start to suspect that person as soon as you hear that.
I'm just... sort of, exploring what makes some mystery stories feel more satisfying than others. Entirely subjective, but then again there are certain titles that repeatedly turn up in a lot of people's top-5 or top-10.