Notes from the Wikipedia entry:
The film is known for two scenes in particular. In one of his most famous shots, Hitchcock starts wide and high, on a second floor balcony overlooking the great hall of a grand mansion; slowly he tracks down and in on Ingrid Bergman, finally ending with a tight close-up of a key tucked in her hand. So arresting is the shot that an outline of the key became a graphic element in the film's promotional material.
Hitchcock also devised "a celebrated scene" that circumvented the Production Code's ban on kisses longer than three seconds—by having his actors disengage every three seconds, murmur and nuzzle each other, then start right back up again. The two-and-a-half minute osculation is "perhaps his most intimate and erotic kiss."..
Claude Rains.. stood three or four inches shorter than Ingrid Bergman... For the scenes where Rains and Bergman were to walk hand-in-hand, Hitchcock devised a system of ramps that boosted Rains's height yet were unseen by the camera. He also suggested Rains try elevator shoes: "Walk in them, sleep in them, be comfortable in them." Rains did, and used them thereafter.