Alternative Character Interpretation: Does Hugo, the ventriloquist's dummy, truly have a will of its own, or is it simply the name and face that Maxwell Frere puts to his Split Personality? Michael Redgrave's performance strongly implies that Hugo is just the dominant facet of Maxwell's Split Personality. For example, in the scene in which Maxwell and Hugo "talk" in prison, you can see Maxwell's lips moving and expressions changing even though it's Hugo who's talking.
He Really Can Act: While Redgrave was already an established and well-respected film star when he acted in Dead Of Night, it was his stunning performance as Maxwell Frere that laid to rest any doubts that he could be as compelling on screen as he was on the stage.
Nightmare Fuel: Sure, the "Haunted Mirror" and "Ventriloquist's Dummy" sequences stand out, and the latter is the reason most people remember the movie, but the overarching narrative with Craig is also truly disturbing. As the audience finishes watching the movie, poor Craig has to go through the whole nightmare again, possibly in real life this time, or perhaps stuck in a "Groundhog Day" Loop.
The very end part with the dummy strangling the architect before he wakes up back into an endless dream loop.