In Notorious there’s a strangely chaste scene of passion in which Ingrid Bergman and Cary Grant hold one another, gaze into each other’s eyes, and declare their love, but only intermittently kiss. Grant even places a phone call during the clinch. The reason is the Hays code, a set of moral guidelines that Hollywood had adopted in 1930. Among other things, it declared that:

The sanctity of the institution of marriage and the home shall be upheld … Excessive and lustful kissing, lustful embraces, suggestive postures and gestures, are not to be shown.

The practical rule was that a screen kiss must not last more than 3 seconds. So Alfred Hitchcock simply toed that line. Bergman later recalled, “We just kissed each other and talked, leaned away and kissed each other again … the censors couldn’t and didn’t cut the scene because we never at any one point kissed for more than three seconds … we nibbled on each other’s ears, and kissed a cheek, so that it looked endless.” In fact it lasted two and a half minutes.