What is the complication in "The Cask of Amontillado"?

In Poe's "The Cask of Amontillado," nearly every part of
Montressor's plan of revenge goes according to schedule, so there are not too many
complications from his angle.  The only real snag is for the reader, as we are not in on
the plan from the beginning.  We are like Fortunato, oblivious to the plan of
revenge.

Getting Fortunato into his catacombs is easy: the
amontillado does the trick.  The complication, I guess, comes in making him walk deeper
and deeper into its dark passages.

For the reader, the
complication comes when Fortunato debates turning back. Fortunato is ill with a bad
cough, and the nitre of the catacombs only makes it worse.  Even Montressor beckons him
to return:

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"Come," I said, with decision, "we will go back;
your health is precious. You are rich, respected, admired, beloved; you are happy, as
once I was. You are a man to be missed. For me it is no matter. We will go back; you
will be ill, and I cannot be responsible. Besides, there is
Luchesi-"

This, of course, is
really part of the plan, verbal irony by Montresor scripted to make it seem like he is a
friend who cares about his health.  This bit of reverse psychology is the only
complication I can see.