Both the stars have subsequently played the title character of Doctor Who: Paul McGann played the Eighth Doctor, and Richard E. Grant the alternate-timeline Ninth Doctor from the animated serial Scream of the Shalka. It's also worth mentioning that Richard E. Grant also played the Doctor in the comedy special Doctor Who: The Curse of Fatal Death. Much later, Richard E. Grant eventually appears within the Eleventh Doctor's tenure as Dr. Simeon / The Great Intelligence. Finally, according to later interviews by Andrew Cartmel, had Doctor Who not been cancelled after the 1989 season, the film's third major actor, Richard Griffiths, would have been offered the part of the Eighth Doctor (after having previously been considered for the part of the Fifth and Seventh).
Paul McGann's character is not named in the film, although the script gives his surname as "Marwood". There is Lady Mondegreenfanon that his first name is "Peter", as many fans have independently and distinctly misheard, "He's just had an audition for rep," as "Peter's just had an audition for rep." The name has somewhat become Ascended Fanon according to the actors and directors involved. Withnail is not named either, but was widely assumed to have the first name of the person he was based on, Vivian McKerrell, and his first name has been stated to be "Vyvyan" by Grant on Twitter.
Corpsing: Richard E. Grant failed to stop snorting during the Penrith Tearooms scene, when talking about the jukeboxes. At the end of the film, Paul McGann can't suppress a chuckle in his voice when he tells Withnail to "stop laughing".
Creator Backlash: Richard Griffiths hated talking about the film, not because of the quality of the movie but because he and most of the cast and the crew were shafted by producer Denis O'Brien and never received their proper payments.
Creator-Chosen Casting: Bruce Robinson cast Paul McGann as Marwood after seeing him on the stage. Then he fired him because he felt that his Liverpudlian accent was wrong for the part. He was recast when Robinson couldn't find anyone else.
Darkhorse Casting: Both Richard E. Grant and Paul McGann had acted on stage and McGann had some success on television, but neither of them had done a film before and were complete unknowns.
Dyeing for Your Art: Richard E. Grant was hired on the provision 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.
The first drink consumed on screen is supposed to be lighter fluid. Vinegar was used in the filming (although it was water in the rehearsals), and the reaction of Richard E. Grant (who's actually teetotal in Real Life) was genuine.
Further to the Enforced Method Acting, Bruce Robinson insisted on getting Grant utterly plastered with him before shooting started. Grant, who's teetotal due to not breaking alcohol down properly, got utterly plastered and, as Robinson put it, finally knew what it was like to be that drunk. Unfortunately, this also made Grant violently sick, requiring a trip to hospital.
Irony as She Is Cast: Richard E. Grant is physically incapable of drinking alcohol, due to his body not producing a certain enzyme, which forces him to throw any alcohol up minutes later.
Something that makes the last twenty minutes even sadder. This was the first film of both Richard E. Grant and Paul McGann. At the time, McGann was an established stage and TV actor, and while this was his first film role, he knew he still had a career waiting for him if his attempt to move into films didn't pan out. Grant, however, had no idea if this was going to be his one and only film or not and so he knew how upset and jealous Withnail was feeling. But as we all know, Grant's fears were unfounded and this film even managed to make him a sex symbol for a while.
Robinson based Withnail on his Real Life friend and actor, Vivian MacKerrell who actually did drink lighter fluid once. Robinson has attributed his friend's early death from throat cancer to this incident.
Real Life Writes the Hairstyle: The last scenes to be shot were those in Regents Park next to the Wolves. Paul McGann wore a wig in the scenes when the two are walking together and talking on the bench.
Referenced by...: The phrase "Even a stopped clock gives the correct time twice a day" was sampled in the intro for Ride's song "Cool Your Boots".
Withnail's grimace◊ complete with a pork pie trying to escape from his mouth was completely accidental.
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.
Word of Saint Paul: Via Twitter, Richard E. Grant said that Withnail's first name is Vivian, the same as the character's inspiration Vivian MacKerrell.
Write What You Know: The film is based on Bruce Robinson's own life in London, albeit condensing two-three years into two-three weeks. As a struggling actor, he lived in impoverished conditions and wore plastic bags as Wellington boots.
Withnail was based on Bruce Robinson's friend Vivian MacKerrell, who later died of throat cancer.
Robinson maintains he received unwanted advances from Italian filmmaker Franco Zeffirelli whilst playing Benvolio in the 1968 film adaptation of Romeo and Juliet. Uncle Monty is loosely based on this experience, and some of his dialogue with Marwood is supposedly quoted verbatim from Zeffirelli's attempts to seduce Robinson.
◊ complete with a pork pie trying to escape from his mouth was completely accidental.
