(spoilers for both knives out movies ahead)
i think what i really enjoy about knives out and glass onion is that they are, while not completely fair-play whodunnits, pretty close to it.
a fair-play whodunnit is a murder mystery which is entirely solvable by the viewer before the detective sums everything up at the end; the viewer is given the same information as the characters, and the same opportunity to figure everything out. this is a style some modern detective stories like to break to preserve the ability to catch the viewer off guard, as hbomberguy elaborates at some length in his sherlock is garbage and here's why video.
knives out gives you almost all the information you need. its possible to figure out on ransom's introduction that there is, at least, something missing from his story, that he returned to the house for some reason after leaving; the dogs were heard barking the night of harlan's death, and he is the only one they are seen reacting aggressively to. likewise, the audience hears nana saying 'ransom, are you back again already?' well before blanc learns about it and realizes its importance.
the only crucial piece of evidence any of the characters ever see that the audience doesn't is the toxicology report, which the audience doesn't get a chance to see before blanc's summation at the end revealing marta's innocence. but even with that omission, it's possible to guess harlan wasn't poisoned! marta lists off the symptoms of morphine poisoning at five and ten minutes on screen, and we see him exhibiting none of them, even after she's left and snuck back in, which must have been more than ten minutes after the initial injection. later we see fran suffering an overdose of the same drug, and she's far more debilitated than harlan was even in his last moments.
glass onion, of course, plays a lot more fast and loose with this concept, because it hides large swathes of the setup from the viewer until the halfway point. blanc actually has a lot more information than the viewer until we get the extended flashback in the middle of the movie.
however, after you know the circumstances of andi's death, like blanc says, you can completely guess that miles killed her! helen even suggests it during one of their first conversations, because it's obvious! of course he did! the only thing the movie does to delay this conclusion is throw out a swarm of red herrings in presenting motive and opportunity for everyone else, but the motive is obvious. the main thing both the audience and blanc need to realize is just that miles is stupid enough to do it. blanc uses his countless malapropisms as evidence when reaching this conclusion, but he doesn't even need to; it's absolutely obvious from the fact, readily available to the audience, that HE MADE HIS HOUSE INTO A BOMB.
likewise, the movie shows you that miles handed duke the drink that killed him, though this is later corrected during his self-serving flashback. you can see the outline of a phone in miles's back pocket after duke's murder even though miles doesn't own a phone, and even a brief shot of him sticking duke's gun in the ice bucket on the table.
additionally, putting a little bit of thought into miles's justification for the lights going out reveals it makes no sense. he was supposed to give a big speech as part of the murder mystery?? no he wasn't! he's dead at this point! he gets shot by the crossbow at dinner! why would he be giving a big speech at 10 pm? because he made up the lights going out on the fly based on blanc's earlier comment, and didn't think it through at all, like everything he does!
i'm not gonna pretend i figured either of these movies out ahead of time on the first viewing- i totally didn't! but i know when the next one comes out, i'm going to be watching very carefully, and probably doing a lot of rewinding.
