I just re-watched the interview with JKR and Steve Kloves on the CoS DVD. Listening carefully, I think they inadvertently gave away some clues.
JKR said that when Steve was writing the CoS screenplay, he nearly left something out that figures largely in book 6. So, from that I can infer that the small bit in book 2 that is big in book 6 is IN movie #2. That means the things that movie #2 left out are NOT the small bit that figures big in book 6. That at least narrows it down.
Further narrowing it down, Steve Kloves said there was a part IN "The Chamber" not in "Chamber" (which is how he refers to CoS) which makes me think the small bit is something that happened or is shown in the Chamber of Secrets itself in book 2. The most obvious guess, to me, would be Harry being the heir of Gryffindor. Any other speculations?
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