Kent Jones said that Brian De Palma declined to participate because of De Palma (2015): "...Noah Baumbach and Jake Paltrow just did a movie about him. They worked on that film for about 4 years. I asked [De Palma] and he said he wanted to save what he thought about Hitchcock for their movie." 
Interpreter Helen Scott (1915-1987) is uncredited in this documentary although her voice is heard throughout. Born in New York, she was brought up in Paris where her father worked for the Associated Press. Decorated for her work on the Free France resistance radio in Brazzaville, Congo, during World War 2, she later worked for the French Film Office in New York and helped Truffaut when he needed help with communicating in English.
Both Hitchcock and Truffaut could actually speak quite adequately in the language of the other, as can be heard in off camera moments. However neither felt confident enough, so they used Helen Scott, a bilingual Truffaut collaborator, to provide simultaneous translation.
Famous writer and screenwriter William Goldman feels that the book this film is about ruined Hitchcock as a filmmaker. He pointed out that it made Hitchcock self conscious and concerned with being an artist in a way that destroyed his ability to engage audiences and be the great director he had been up to that point.