1. Hitchcock likes to stimulate his audience's curiosity and reasoning. That's why he reveals that there are secrets, beyond what the narrative of his films is capable of passing. With this, it shows that you have the perception that your audience is able to see beyond what you are watching and that they will create ideas and theories that will further increase the audience of your work. In other words, he sees his audience's curiosity as the driving force behind his work.
2. Hitchcock wrote to promote curiosity and create mysterious narratives where the audience needed to work on their reasoning to reach a conclusion. Because of this, we can say that this sentence is directly related to Hitchcock's purpose for writing. That's because when he says that the audience will work like the devil to discover the invisible secrets for the characters, he means that the audience will have a lot of work to unravel the secrets of his narrative.