It's only a minor exaggeration to say that the whole script could be put here, directions included. Still, exercising restraint, here's a few of the highlights:
Alone in the cottage way out in the countryside, they think that someone is trying to break in and kill them. The suspense builds and builds... then at the hight of suspense, it is revealed to be porcine Uncle Monty dropping in unannounced. There is a long, long, pause before Withnail exclaims, "MONTY YOU TERRIBLE CUNT!".
Withnail is a gold mine of comedy:
"We've gone on holiday by mistake!"
"Where's the booze? I DEMAND TO HAVE SOME BOOZE!"
"I want something's flesh!!" Followed by a hysterical scene of Withnail splashing about in a stream, blasting fish with a shotgun.
"I feel like a pig shat in my head."
"We want the finest wines available to humanity and we want them here and we want them now."
Then there's the immortal: "I have a heart condition. If you hit me, it's murder."
Withnail: How dare you. How dare you call me inhumane! Right, you fucker! I'm going to do the washing-up! Marwood: No! You can't! It's impossible! I've looked into it!
Just the sight of Withnail ranting away while dressed in nothing but underpants and a blue rubber glove, covered in Deep Heat.
I'M A TRAINED ACTOR, REDUCED TO THE STATUS OF A BUM!
Marwood AKA "I" has a few gems as well:
"A coward you are, Withnail. An expert on bulls you are not!"
"My thumbs have gone weird! I'm in the middle of a fucking overdose! My heart's beating like a fucked clock!
While in the team room, as Withnail is raving, Marwood is just happily eating and asking for cake.
After Withnail's "finest wines available to humanity" like, Marwood just nods in agreement, a big smile on his face.
When Withnail keeps insistently demanding to know whether the farmer — who is dressed like a farmer, driving a tractor, and overall looking exactly like a farmer — is in fact the farmer:
Marwood: STOP SAYING THAT, WITHNAIL! OF COURSE HE'S THE FUCKING FARMER!
And Uncle Monty is a treasure trove of quotes:
"As a child I used to weep in butcher's shops."
"Flowers are essentially tarts. Prostitutes for the bees."
"There is, you'll agree, a certain 'je ne sais quoi' oh, so very special about a firm, young carrot."
"It's trying to get itself in with you. Trying for even more advantage. It's obsessed with its gut. It's like a bloody rugby ball now. It will die, it will die!"
Danny the drug-dealer:
His whole getup is laugh-out-loud hilarious.
Every time he tells Withnail not to get "uptight."
Withnail says he was turned down for a cigarette commercial because "a gang of cheroot vendors considered a haircut beyond the limit of my ability."
Danny: I don't advise getting a haircut, man. All hairdressers are in the employment of the government. Hair are your aerials. They pick up signals from the cosmos and transmit them directly into the brain. This is the reason bald-headed men are uptight. Withnail: What absolute twaddle.
"If I medicined you, you'd think a brain tumor was a birthday present!"
His story about Presuming Ed and his court, when the judge reprimanded him for not being properly dressed and that court is not a fancy dress party. Funny when one considers the huge white wigs that judges wear in Britain.
Danny: (as Presuming Ed) And you think you look normal, your honour?
His plan about starting a business with dolls that would shit themselves, based on already existing dolls that piss themselves. It's disgusting, to be sure, but little girls seem to like it.
Then there's the second policeman who pulls Withnail over for drunk driving, who only gets one line, but what a line it is: "Getinthebackofthevan!!"
The scene where Marwood and Withnail, totally wasted, pretend they're millionaires in a cafe full of posh people. Richard E. Grant breaking character and not being able to stop giggling makes it all the more amusing.