What makes the reader interact with the characters, & what responsibility do we then share to shape society through observation & innovation.in The...

Wow, what a thoughtful question!  I'll admit that I really
had to think about this one!  My final assessment is that it's not a "what" that makes
the reader interact with the characters of this novel, it's a "who."  Namely, it's Nick
Carraway that makes us interact with the other characters.  He is our narrator (some
claim him to be a totally unreliable one, actually), and it's through his eyes that we
see everyone in the story.

It is Nick's interaction with
Gatsby that continually confuses me.  The reason is that Nick is continually conflicted
about that particular character in the first place.  I'll give you an easy example. 
There are places in the novel where Nick has a bad feeling about Gatsby, such as when
Gatsby is telling his life story to Nick who notices, "his whole statement fell to
pieces, and I wondered if there wasn't something a little sinister about him, after all"
(65).  Yet, Nick's confusion is apparent when one starts citing the positive things Nick
says about Gatsby:

readability="11">

"They're a rotten crowd," I shouted across the
lawn.  "You're worth the whole ... bunch put
together."

I've always been glad I said that.  It was the
only compliment I ever gave him, because I disapproved of him from beginning to end.
(154)

This doesn't say much,
I suppose, about what Nick thinks about the other characters in the
story as well (if Gatsby, a man Nick "disapproved of" is worth the "whole ... bunch put
together").  Such continues the air of mystery surrounding that character, clouding our
entire vision of Gatsby as well as 1920s New England.

Then
comes the true rub of your question, what responsibility do we share with Nick through
observation and innovation?  Hmmm.  I suppose we are required, then, to learn from
Gatsby's story.  We learn not to repeat his failures.  We learn that the American Dream
need not be found in glittering mansions, but elsewhere (perhaps in thriving, happy,
middle-class, Midwestern families with strong values?).  We learn to neglect the
recklessness that became the signature action of the 1920s and, perhaps, learn to
espouse deeper morals that will guide us safely to a long and happy life full of
meaning, ... not a short desperate life full of riches and
obsession.