"Fork It!" was the line that secured Richard E. Grant the part of Withnail. Director Bruce Robinson knew in his head exactly how the line should be uttered by the actor (as with the Policeman's "Get in the back of the van!"), and Grant delivered it exactly how he envisaged without being prompted.
In the scene where Withnail (Richard E. Grant) ostensibly downs a bottle of lighter fluid, the can, which in rehearsals had been full of water, was full of vinegar. Director Bruce Robinson used vinegar on the take to get a better facial reaction from Grant. The vomiting was scripted and faked.
The first preview screening appeared to be a total disaster - the audience sat there stony-faced, never laughing once. It was only after the screening had concluded that a distraught Bruce Robinson discovered that the audience was comprised entirely of non-English speaking German tourists who were all staying at a hotel nearby.
Withnail is a ferocious drunk, but he was played by the teetotaler Richard E. Grant. Finally convinced that he needed to get drunk at least once to have the proper insight into the character, Grant "filled a tumbler with vodka and topped it off with a bit of Pepsi", then swilled the whole thing down. He was teased the next day by costar Paul McGann and director Bruce Robinson, who assured him that he would never be so funny on film again.
When Bruce Robinson was appearing in Franco Zeffirelli's Romeo and Juliet (1968), the gay director tried constantly to seduce him. Robinson incorporated many of Zeffirelli's chat-up lines into Uncle Monty's dialog as he pursues Marwood.
Ralph Brown turned up for his audition dressed up like the Danny described in the script, with purple hair and painted fingernails. The casting director Mary Selway and director Bruce Robinson found him frightening but also hilarious. Because of his extra mile, he got the part.
Originally written by Bruce Robinson as a novel. Its conclusion is quite different from the film's: after his soliloquy in the rain at the park, Withnail returns to the flat he shared with Marwood, loads the rifle he took from Monty's country home, pours some wine (also taken from Monty) down its barrel, then puts the muzzle to his lips and drinks. He then pulls the trigger on the gun, killing himself.
In the tearoom scene, Richard E. Grant breaks out laughing. This wasn't scripted, but every time he spoke, he could hear the snorting of the dogs belonging to the old ladies at the table behind. He thought that this was someone laughing and kept corpsing. After too many re-takes, the director gave up and kept the laughter in.
During the motorway scenes, in the interior shots of the car, Paul McGann is seen to be driving, but in some exterior shots, the driver is Bruce Robinson, the director. The reason for this is that McGann had only just passed his driving test when the film was made, and so was a bit wobbly on the motorway. Also, as he pulls away in London to set off for the Lake District, he stalls the car. This was unintentional, and was included anyway.
Despite Paul McGann's speculation on the DVD commentary that Withnail might have gone to Stowe, both he and Uncle Monty are supposed to be old boys of another public school: Harrow. The first hint of this is the print of Harrow hanging up in the Camden flat (it's the one of the redbrick building just next to the door frame leading to the kitchen), but the real clincher is when later, at Monty's house, Withnail lies that Marwood went to "the other place", to which Monty replies, "Oh, you went to Eton!" Members of Eton and Harrow often refer to pupils of the other in this manner.
Although credited on screen only as "...and I", Paul McGann's character is named as "Marwood" in the script. It is widely believed that the character's first name is Peter; although this is incorrect. This myth arose as the result of a misheard line of dialogue when Withnail and Marwood are enjoying drinks at Monty's house. The only occasion Marwood's name appears in the film, is when it can be just be made out written upside down on the envelope he receives at Crow Crag.
During the film, Danny mentions an acquaintance of his named The Coalman, then threatens Withnail with a potent drug called The Embalmer. Both The Coalman and The Embalmer are the names of two hangover "cures".
The name "Withnail" came from man called Jonathan Withnall that Bruce Robinson knew as a little boy. He was a local character who reversed his Aston Martin into a police car in a pub car park. Robinson said he's never been able to spell very well, and spelled his last name "Withnail" by mistake. The character of Withnail was based on Robinson's close friend, actor Vivian MacKerrell who died of throat cancer in 1995. They met at The Central School of Speech and Drama in 1964. The pair lived with David Dundas, Michael Feast and others in a Victorian townhouse bought by Dundas's parents in Camden Town, London.
Handmade Films kept back £30,000 of Bruce Robinson's fee to pay for the scenes when Withnail and Marwood are driving through the rainstorm to get to Uncle Monty's cottage. The producers didn't believe these scenes were needed, but the director considered them essential. He was never reimbursed his money after the film's success.
Richard E. Grant was hired on the proviso that he lost weight. Ironically he had just been trying to gain some weight through working out because he thought was too skinny. He went on a diet of protein shakes to achieve his gaunt look in the film.
In the scene where they set off for Penrith, in the shot where the Jaguar drives away after Marwood flips the shades down on his glasses, Bruce Robinson's rare Aston Martin DB4 convertible can be seen parked in the background. This car also appears in the Bagleys' garage in How to Get Ahead in Advertising (1989)
Both Paul McGann and Richard E. Grant went on to play the Doctor in Doctor Who. McGann was the 8th Doctor and Grant was the 9th (in the 2003 animated webcast Doctor Who: Scream of the Shalka (2003)), though his status was changed to "unofficial" when the new series of Doctor Who started in 2005. He also played an incarnation of the Doctor in the Comic Relief skit Comic Relief: Doctor Who - The Curse of Fatal Death (1999). Coincidentally, Richard Griffiths was once strongly considered to succeed 7th Doctor Sylvester McCoy, but this was never realized after the series was canceled in 1989 and McCoy's Doctor later regenerated into Paul McGann's in the 1996 TV movie. Griffiths had also previously been considered to succeed 4th Doctor Tom Baker in 1981.
The Barman in the London pub. He also doubles up for Richard E. Grant as the driver of the car on the way back from London (and can be seen at a couple of points) and Michael Elphick's poacher character in the long distance shots of him visiting Crow Crag.