The desperation in Mr. Green as he attempts to prevent the cop from discovering all the corpses, from his ridiculous Captain Obvious tour of the house, to his Obfuscating Stupidity in feigning ignorance of which rooms the cop is talking about (made funnier if you take the third ending into account—he's a real cop going through ridiculous antics to prevent the sting from being ruined), culminating in his reactions to what they discover in the study and the lounge. Michael McKean's expressions have to be seen to be believed. "Because... because it's all too shocking!"
Mrs. White admitting that she killed Yvette, and explaining why— Madeline Kahn ad-libbed the gem with the director's foreknowledge. None of her costars knew about it, however, and their reactions when to her spiel are priceless.
In the second ending, you see he's gotten wise and ducks out of the way when he demonstrates how Yvette and the Cop were killed.
The gagging noise Mrs. White makes when Wadsworth pretends she's Yvette and strangles her is priceless. (As is her little scream when Mr. Green commiserates with her, "I hate it when he does that!")
When Wadsworth lies face-down on the floor in the study to simulate Mr. Boddy, he appears to be dead. Everyone gasps and Mr. Green exclaims "Oh, grand!" before Wadsworth springs back up again, nearly giving poor Mr. Green a heart attack.
Miss Scarlet, however, just calmly lights up her cigarette in the darkness.
When Wadsworth falls out of the meat locker in the Kitchen, Mr. Green catches him, then simply drops him onto the floor in exasperation.
Even better, Wadsworth continues his exposition without missing a beat from his position on the floor.
When Wadsworth leaps out from behind the secret passage painting in the study and chases Mr. Green with an imaginary candlestick to re-enact Boddy's death, listen closely: he actually does a laughter version of "Na na na-na na!"
Wadsworth shooing Mrs. Peacock to her chair during the explanation, managing to get her to squawk to reenact her screaming.
Mr. Green walking nonchalantly out of the bathroom after Wadsworth has thrown him in there (simulating the killer hiding Mr. Boddy's body), drying his hands on a towel and accompanied by the sound of a toilet flushing.
A good portion of the humor during The Summation comes from the madcap soundtrack, which unsurprisingly is quite vaudevillian in nature.
The first ending.
Wadsworth: The game's up, Scarlet. There are no more bullets left in that gun. Ms. Scarlet: Oh, come on, you don't think I'm gonna fall for that old trick? Wadsworth: It's not a trick. There was one shot at Mr. Boddy in the Study; two for the chandelier; two at the Lounge door and one for the singing telegram. Ms. Scarlet: That's not six! Wadsworth:1 + 2 + 2 + 1. Ms. Scarlet: Uh-uh, there was only one shot that got the chandelier. It's 1 + 2 + 1 + 1. Wadsworth: Even if you are right, that would be 1 + 1 + 2 + 1, not 1 + 2 + 1 + 1. Ms. Scarlet: ...'kay, fine. 1 + 2 + 1 — SHUT UP! The point is, there's one bullet left in this gun and guess who's gonna get it!
When Miss Scarlet and Colonel Mustard are locked in the lounge with the dead motorist, and the others are trying to get the door open.
Professor Plum: The door's locked! Mr. Green: I know! Professor Plum: Then unlock it! Mr. Green: Where's the key? Wadsworth:(after searching his pockets) The key is gone! Professor Plum: Never mind about the key! Unlock the door! Mr. Green:(grabbing Professor Plum and shaking him) I can't unlock the door without the key! (releases Professor Plum and bangs on the door) Let us in! Let us in! Miss Scarlet and Colonel Mustard: Let us out! Let us out!
Then when Yvette shoots the lock to open it Col. Mustard staggers from the door screaming "I've been shot!" despite being perfectly fine.
Even this throwaway line, from after the door is opened:
Mrs. White: How did you get in? Mr. Green: The door was locked! Mrs. White: It's a great trick!
The guests come back to the study with the dead cook, only to discover Mr. Boddy's "body" has vanished. Several of them immediately drop Mrs. Ho's front half with a very resounding thud. Then comes this exchange:
Mrs. Peacock: What are you all staring at? Mr. Green: Nothing. Mrs. Peacock: Well who's there? Colonel Mustard: Nobody. Mrs. Peacock: What do you mean? Wadsworth: Nobody. No Boddy, that's what we mean. Mr. Boddy's body, it's gone! Mrs. White: Maybe he wasn't dead. Professor Plum: He was. Mrs. White: We should have made sure. Mrs. Peacock: How? By cutting his head off, I suppose. Mrs. White: That wasn't called for.
The guests trying to make the dead bodies seem alive for the benefit of the cop. Especially Miss White making out with the corpse on the sofa!
"To make a long story short—"
Colonel Mustard's investigation of the kitchen culminates in him opening a closet, suspecting a body or the murderer or anything, diving his head forward to make sure nothing escapes him... only to get bonked on the head by a pull-out counter.
He also manages to hit his head on the underside of the dining room table after the power is turned off.
Mrs. White: Yes, just the five. Husbands should be like Kleenex: soft, strong, and disposable.
Colonel Mustard: You lure men to their deaths like a spider with flies!
Mrs. White: Flies are where men are most vulnerable.
Colonel Mustard: Right!
The guests becoming less and less concerned for the care of the corpses as the movie goes on. When the cook dies, they take great care not to aggravate the deadly injury. By they time they get to the singing telegram girl, they simply drop her face-first on the floor from waist-height.
Mr. Green and Yvette have been sent up to the attic to search for the murderer. Upon reaching the steps, they both stay frozen at the bottom, staring up into the attic for quite some time (even still doing so after a cut-away and back with Miss Scarlet and Colonel Mustard), despite Mr. Green unconvincingly stating there wasn't anyone up there. When next shown, Yvette tries to reassure him by saying she'll be "right behind him" ("That's why I'm nervous"), leading them to finally decide to go up the narrow staircase together side by side. The ridiculously awkward way they manage this (complete with quite the Panty Shot from Yvette) has to be seen to be believed.
Prior to that, they're getting themselves set apart in pairs in the kitchen. Professor Plum is looking around finding his match, and it's Mrs. Peacock. What he says to her is priceless.
Professor Plum: It's you and me, honeybunch. Mrs. Peacock: Oh, God...
Similarly, Plum and Peacock at the top of the basement stairs, understandably reluctant to go down:
Prof. Plum.: What are you afraid of, a fate worse than death?
Mrs. Peacock: No, just death. Isn't that enough?
In the study, after Mr. Green has stopped Mrs. Peacock from screaming over the possibly-poisoned brandy, they all examine the broken glass and decide they'll never know if it was poisoned. "Unless," Mr. Green points out, "she dies too." Cue the guests running over to surround Mrs. Peacock and intently study her for signs of impending death, while she stares at them like they're utterly mad.
The discussion of Mrs. White's husband and his untimely end:
Mrs. White: He was found dead on the floor, his head had been cut off, and so had his...you know...