Focus on the physical setting of "The Storm" and discuss its impact on Calixta and Alcee's actions.

The story, set in the south in the middle of a torrential
downpour, is all-important in being the catalyst for the action that occurs.  First of
all, if there was no storm, Bobinot and Bibi would have been home a lot sooner than they
were, and the opportunity for Calixta and Alcee to act would have been cut short. 
Secondly, if it weren't for the storm, who knows if Alcee would have even stopped at the
house at all.  Then, if it weren't for the storm, he at least probably wouldn't have
come inside.  He planned, originally, to stay outside on the porch, but, as Chopin
writes,

"it
was soon apparent that he might as well have been out in the open: the water beat in
upon the boards in driving sheets, and he went inside, closing the door after
him."

So, even if it had been
a light shower or storm, he could have stayed outside, and they might not have
succombed.  But, the rain is intense, driving in at a sideways slant, so he has to come
in.

Then, note the arrangement of the house.  As soon as he
steps in, he can see into her bedroom.  Chopin probably planned that on purpose; seeing
that room suggested things to both of their minds that further tempted them both.  The
lightning and thunder then drove them into each other's arms, where the temptation was
too much to bear.

The storm was the critical factor in this
story, and the floor plan of Calixta's house.  If any of those had changed, they
probably wouldn't have acted the way that they did, that day.  Chopin does hint, at the
end of the story though, that Calixta and Alcee find great happiness in each other, and
even become better spouses because of their tryst; so, maybe eventually they would have
each strayed in their own way, since it seems they weren't happy in their stations. But
for the events of this story, the storm and setting were crucial.  I hope that helped;
good luck!