remember - these things are based on a trillogy of books. They are technically one big book - but were published over time.
the movies are just taken exactly from the books - so since there is no prologue or recap in the books, then there will be nothing in the movies either.
in the books, it is done like most stories - the initial part is there to establish character identity and expose the theme of the rest of the book.
the second part of the book is how they then move towards the end goal and exposes the actions on the motivations established in the first part.
and then the last book obviously has to contain the climax and then the denouement.
much in the way that Barry Manilow has a basic format for writing hit songs - pretty much any non-fiction writer follows the same basic theme as well - but the lord of the rings is long, so it is spread out more than the average pulp fiction.
also, I think it is great that the director is basically just translating the books directly to the screen and not dumbing it down the hollywood way (so that they don't do what you were mentioning and have explainations and flashbacks in there to makeup for the people that didn't see the others).
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