Question: Very last scene after they turn the car around and you see the back of their heads, to me it doesn't look like Cary Grant or Joan Fontaine? Is it them or their doubles?
Answer: Hitchcock had a different ending on the movie, but the studio and test audiences hated it. Grant ends up splitting up with Fontaine. Hitchcock was pressured into a happy ending, but weeks had passed since the end of filming and the two popular stars were off on other projects. Two actors were hired to as stand-ins for the added footage of the car turning around and the two going happily home together.
Answer: It could have been body doubles, who are often used for filler scenes like this, but there's no way to know for sure.
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