Joan Fontaine liked the character of Lina in Suspicion (1941) so much that she sent Alfred Hitchcock a note after she read the novel ("Before the Fact", by Anthony Berkeley) offering to play the part for free, if necessary.
Joan Fontaine's performance in this movie is the only Oscar-winning performance that Alfred Hitchcock directed.
In the scene where Johnnie brings a glass of milk up to Lina, Alfred Hitchcock had a light hidden in the glass to make it appear more sinister:
From IMDb: Grant is a perfect choice to play Johnnie Aysgard. He has the dark, handsome looks, that gleaming smile and loving charm and he literally sweeps spinster Lina McLaidlaw (Joan Fontaine, Oscar winner for this role) off her feet. His presence only vaguely suggests the menace hidden underneath and this is perfect for a convincing psychological, cerebral thriller. If Lon Chaney, for example, had played Aysgard, or Joseph Cotten, Orson Welles, or even Basil Rathbone for that matter, it wouldn't be hard to yell at the screen and pinpoint the villain in the story. Grant, however, is so completely at home in his ambiguousness that even in the climactic scene where he drives Fontaine to her mother's home, we still can't quite decide what his intentions are even though every added piece of evidence leads to the mounting horror that he is about to kill her.