A last-minute cast change has forced the National Actors Theater to postpone the opening of its revival of the British mystery thriller ”Night Must Fall,” starring Matthew Broderick.

J. Smith-Cameron has stepped in for Pamela J. Gray, who was fired yesterday by director John Tillinger.

Production officials cited ”creative differences,” but insiders said the problem was a lack of chemistry between Gray, who was making her Broadway debut, and Broderick.

”Night Must Fall,” currently in previews, was slated to open officially on Sunday night at the Lyceum Theater. Its new opening date is March 8.

The 1935 melodrama, by Emlyn Williams, centers on a young man (Broderick) who befriends and then plots to kill a rich, older woman, played by English actress Judy Parfitt.

Smith-Cameron, who won acclaim two years ago for her portrayal of magazine editor Alexa Vere de Vere in the hit off-Broadway play ”As Bees in Honey Drown,” will play Olivia, the woman’s niece, who suspects that the Broderick character is up to no good.