SELSEY PAVILION. Drama of the highest order and of unusual character is provided by 20th Century-Fox in "ANNA AND THE KING OF SIAM," co-starring Irene Dunne, Britain's Rex Harrison and Linda Darnell. The film will open on Monday at The Pavilion. Margaret Landon's best-seller provided the story, an account of an Englishwoman's courageous endeavours to bring learning and humanity into the luxurious, but basically barbarous, court of King Mongkut of Siam. Rex Harrison makes the role of the King something memorable indeed. Anna is played by Irene Dunne, and her dignity and gracious character have never been seen to finer effect. On Thursday, Lizabeth Scott is starred with Humphrey Bogart for the first time in John Cromwell's "DEAD RECKONING", a Columbia production.