Walter Murch — no stranger to montage — is describing a sequence of shots. He’s matter-of-fact but speaks with a surprising curiosity. He notes an odd cut; it’s one of the famous cuts, though it’s not famous for being particularly unusual. Then he half-pauses, as if to question his own judgement. It’s a nicely observed moment, maybe even the best, in the docu-essay 78/52. Rather than have Murch continue to narrate the sequence, though, director Alexandre O. Philippe subs him out for a South...