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Factual error: When they are driving back home from the holiday cottage you can clearly see an M25 sign in the background. The M25 didn't exist in the 60s.
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Continuity mistake: Whenever you get a shot of the car travelling along the motorway or road, the cars going in the same direction as the Jaguar are all true to 1969 - the year the film is set - but the cars travelling in the opposite direction or parked by roads are all contemporary with the time it was made (eg. Ford Sierras).
Chosen answer: I'm not sure they are friends, they were just thrown together by circumstance. Withnail treats Marwood badly throughout the film. When Marwood gets his role, he smartens himself up and can't wait to get out of there, not even bothering to move his stuff out, saying his dad will do it. I think that they both realise that it's the end of their relationship, and Marwood just wants to get it over with.
Madstunts