Not that we mind waiting, with a cast this diverting. Indeed, the mystery itself is the least interesting aspect of the film. What intrigues us are the fraught social dynamics, the class tensions between servants and masters, the subtle, nasty games of one-upmanship played by the guests of the powerful Sir William McCordle (Michael Gambon) and his icily soignee wife, Lady Sylvia (Kristin Scott Thomas).

Among the invited are the preposterously snobbish Countess of Trentham (Maggie Smith, venomously entertaining), the film star Ivor Novello (Jeremy Northam), a Hollywood producer (Bob Balaban) attended by his “Scottish valet” (Ryan Phillippe) and a gaggle of aristocrats more desperate than their titles would suggest. (Claudie Blakley is particularly touching as the neglected wife of the blackmailing James Wilby.) The below-stairs staff is even more star studded: Helen Mirren, Alan Bates, Emily Watson, Clive Owen, Kelly Macdonald and Eileen Atkins among them. Not surprisingly, the movie’s sympathies are with the servants, whose characters are allowed more than the one-dimension allotted to most of their masters. A lovely lyrical moment encapsulates the class difference: as Novello sings in the parlor to a room of bored, indifferent guests, the staff huddle together to listen to his music in awed appreciation.

The ensemble work is delicious, the production sumptuous. It’s Altman’s most assured movie since “Short Cuts.” But for me it never quite came to a full boil. Perhaps because he’s dealing with the English upper classes, not known for their spontaneity, “Gosford Park” doesn’t have the astonishments and behavioral surprises of Altman’s very best films. When Altman’s on fire, you can’t be sure where his movies might take you. “Gosford” is a fine, well-groomed entertainment, but the road it takes has already been well paved.

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