Alfred Hitchcock appears in almost all of his movies in a small, non- speaking, cameo role. The Hitchcock film 'Lifeboat' is about the survivors of a merchant ship and a U-boat that sunk each other and the entire movie is set at sea, in the lifeboat, with the entire cast present in the boat from the beginning to the end of the film (not everyone survives but what I mean is that the cast is not variable and does not include any incidental people). This provided a problem for Hitchcock's cameo role as there was no way for an additional person to wander into and then out of the movie's setting. He considered floating by as a body but eventually settled on a pretty clever scheme I think: there was a newspaper on board the lifeboat and as Hitchcock had recently lost a great deal of weight, he appeared in an advertisement in the newspaper as both the 'before' and the 'after' photo showing the performance of a weight loss drug.
Brian