In the reversed opening scene, the footage of the shell casing reversing is really writer/director
Christopher Nolan blowing on it, since it would have been too difficult to try to get the shell casing in a close-up view as it hit the ground, while keeping the camera still. In order to get a better looking scene in reverse, they didn't simply film it forward and reverse it in editing. They actually fed the film through in reverse, making a crisper picture. However, this specific shot was not meant to be filmed in reverse. It was done accidentally, and so they had to, in editing, reverse it. That means it's a shot of a simulated reversal, recorded backwards, and then manually reversed to get the desired effect.
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