As is always his style in his ensemble films, Altman has his cameramen prowl around a set in which numerous events are unfolding simultaneously; he often seems to concentrate on the most trivial (if absorbing) activities, while the important stuff is going on in the background or solely on the soundtrack. It's a canny technique, particularly for a mystery: We never know whether the highlighting of certain things is important or not. When the camera adopts such an indiscriminate eye, clues can be dropped in without being obvious.
Altman's technique also allows his huge cast to act up a storm, in the best sense.
Gosford Park has roughly half the best actors in England in it; in addition to those mentioned above, it also features Derek Jacobi, James Wilby and Clive Owen. (American heartthrob Ryan Phillippe is in there, too, and perfect for his role.) Much of the beauty of what the actors provide is not apparent on a single viewing; second time around, armed with foreknowledge of who these people are and what is going to transpire, one can perceive and understand numerous little expressions and reactions whose meanings weren't apparent the first time.
It really is amazing that Altman has endured this long, through all his career ups and downs. He's 76 now, and I can't think of another American director who has still turned out first-rate work at that age. (Hitchcock was 72 when he made Frenzy; Cukor, 65 for My Fair Lady; Wilder, 72 for Fedora, which was only first-rate for part of its length.) If Altman's last effort, Dr. T and the Women, was one of his most critically reviled films ever, Gosford Park should do much to salvage his reputation.