Beatrice: Against my will, I am sent to bid you come in to dinner.
Benedick: Fair Beatrice, I thank you for your pains.
Beatrice: I took no more pains for those thanks than you take pains to thank me. If it had been painful, I would not have come.
Benedick: Ha! “Against my will I am sent to bid you come in to dinner.” There’s a double meaning in that. “I took no more pains for those thanks than you took pains to thank me.” That’s as much as to say, “Any pains that I take for you is as easy as thanks.” If I do not take pity of her, I am a villain.
Benedick trying to justify going after Beatrice, despite having sworn that he's way too cool to love or get married:
"The world MUST BE PEOPLED!"
Shortly after Benedick's rant about his perfect woman, Don Pedro and Claudio come into the garden to trick him into loving Beatrice. Benedick's reaction to his lovesick buddy is priceless, especially in the hands of Kenneth Branagh.
Benedick: Here comes the prince... and Monsieur Love. I will hide me in the arbour.
Followed not thirty seconds later by Don Pedro saying (well, practically yelling) "Come hither, Leonato! What was it that you told me of today? That your niece Beatrice was in love with Signor Benedick?!" *Cut to Benedick sitting down too quickly, breaking his chair and thwacking his head off the ground*
Really, the fact that they're all bellowing their lines and being really obvious about it is hilarious.
Don Pedro: What effects of passion shows she? Claudio: Bait the hook well - this fish will bite! *Pause while Leonato stammers and looks absolutely lost* Leonato: What... effects, my Lord? *Don Pedro smiles confidently with a little nod... and Leonato keeps stammering. Don Pedro starts glaring at Leonato* Leonato: ...you heard my daughter tell you how! Claudio: She did indeed! Don Pedro: How, I pray you? *Pause while they go into a huddle and Leonato whispers urgently, as Benedick looks on wide-eyed* Don Pedro: You amaze me! *Benedick dives into the bushes*
Bonus points for Don Pedro saying that Benedick has "a contemptible spirit", to which Benedick replies with a loud and indignant noise... which he then tries to cover up by making bird calls.
Emma Thompson's dash between the shrubbery and statues as Beatrice eavesdrops on Ursula and Hero's conversation is pretty funny, too.
Doubly so for the ass rant, since Dogberry is so disgusted by it that he spends several seconds trying to pull on his jacket, not noticing that it's far too small to fit him. He pauses in his rant to notice that he actually grabbed the jacket of his much smaller co-worker.
In the Whedon adaptation, the announcement that Dogberry caught Don John is made all the funnier since we watch him awkwardly frisking the guy via a video camera. Doubly so for Firefly fans, who will notice that Mal is frisking Simon.
Nathan Fillion is all well and good, but Michael Keaton runs rings around him. Keaton has repeatedly said in interviews that he was terrified of playing Dogberry, and also said that he was sick and running a high fever during most of the filming. Doesn't stop him from riding an invisible horse around and having one of the best moments of the "I am an ass" speech. Bonus points for someone insulting him earlier in the scene and his men responding with an "Oooooh!"
The invisible horses get a comeback during his last scene... He and Verges take off down a road - and then about two seconds later you hear several dogs barking and they go zipping off in the opposite direction!
Joss Whedon's adaptation really takes the slapstick of Don Pedro's Zany Scheme up to eleven. Benedick executes completely unsubtle ninja rolls to eavesdrop on Claudio and Leonato, and Beatrice's reaction to overhearing that Benedick loves her is to pratfall down a short flight of stairs, then dive behind a kitchen counter and whack her head on the underside. It looks painful, but boy is it funny.
In the Joss Whedon adaptation, when Benedick has completely fallen for Beatrice, his friends walk in on him lying on his bed, grinning and staring at her picture like a love-struck teenager. Their response is a subtle fist bump.
The Dogberry in that version is hysterical as well - he's a Boisterous Bruiser who secretly wants to be Rambo. Hell, the show goes to intermission when he comes out with a huge shotgun and gleefully pumps it.
During the above-mentioned Zany Scheme, Benedick ends up covered in white paint and throws a book across the stage, while Beatrice somehow gets hooked to a cable and raised several feet into the air while she paddles madly around (and covers up her chest)! It Makes Sense in Context.
Plus, Catherine Tate corpsing and telling the audience to quiet down for a good few seconds, then noticing her shirt is still hanging low.
At one point, Benedick is taking a swig of beer when the Duke says something that causes him to spit-take and spray beer all over the back of Leonato's head and the Duke. The trio then awkwardly pretend to check if it's raining in order to keep the deception going.
Joss Whedon's commentary brings up how, given what a gullible idiot Claudio is for most of the play, some audiences thought his line "Another Hero" indicated that he really did think he was looking at another woman identical to Hero.