During group scenes, director Robert Altman had two cameras going at all times, moving about (out of each other's shot, of course). His intention was to prevent the actors from acting to the camera but instead to play the scene more naturalistically.
When Jeremy Northam's character plays the piano, it is either him playing, or his brother, who is a classically trained pianist.
Rather than just use a typical boom mike to pick up dialogue, director Altman had all the actors wear portable microphones to assist in creating overlapping dialogue. A technique he first developed during A Wedding (1978) and has used several times since.
The jewelry worn by the upstairs ladies in the film was all authentic and had to be escorted in by armed guards each day.
The camera is always moving (if only slightly) in every shot of the film as requested by director Robert Altman.
The hunting scene is a direct reference to the infamous hunting scene in Jean Renoir's Règle du jeu, La (1939) (The rules of the game). Renoir's story is about some aristocrats in a country house, a murder, and the interaction they have with their servants.
Eileen Atkins, who plays Mrs Croft the cook, was co-creator (with actress Jean Marsh) of the classic British drama series "Upstairs, Downstairs" (1971). The movie also features Meg Wynn Owen, who starred in the series from 1973-1975
Ivor Novello (played by Jeremy Northam) was a well known London matinée idol, singer and composer who starred in Alfred Hitchcock's silent classic The Lodger (1927). He had a good voice and starred again in the successful 'sound' remake (1932). In Gosford Park, set in November 1932, Countess Constance (Maggie Smith) refers to the remake as a flop.
There is always a servant present in each scene.
There really was a Charlie Chan in London (1934) film made in 1934 and it was indeed a mystery set in an English manor house. While it did feature Alan Mowbray and Ray Milland, it was produced by John Stone, not Morris Weissman (a fictional character).
None of the actors who played servants wore any make-up.
In the DVD commentary, director Robert Altman states he included the F-word several times on purpose to get an R-rating because he didn't want kids to see the film - he thinks kids wouldn't like the film so he wanted to keep them out (especially 14-year-old boys).
The name Gosford Park is never once said in the film.
Inspector Thompson never gets a chance to introduce himself properly.
Altman consulted the writer Ezna Sands in depth on the idea before commencing with the project, having wanted to employ his doctoring skills on the script. Sands simply said it was as close to perfect as it could possibly be.
Ryan Philippe was cast at the 11th hour replacing an actor who withdrawn.
Kenneth Branagh was first choice for Inspector Thompson but had to decline owing to a scheduling conflict.
Cast members Alan Bates and Derek Jacobi have both played Danish King Claudius, in _Hamlet (1990)(I)_ and Hamlet (1996), respectively.