The Birds (1963)

Alfred Hitchcock saw Tippi Hedren in a 1962 commercial and put her under contract. In the commercial for a diet drink, she is seen walking down a street and a man whistles at her slim, attractive figure, and she turns her head with an acknowledging smile. In the opening scene of the film, the same thing happens as she walks toward the bird shop. This was an inside joke by Hitchcock.