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[Marlow on the brickmaker]: "I let him run on, this papier-mâché Mephistopheles, and it seemed to me that if I tried I could poke my forefinger through him, and would find nothing inside but a little loose dirt, maybe." (1.61)
Not only do we know that the brickmaker's words are empty, but Marlow describes him as a "papier-mâché" figure, implying that he's hollow inside. (And maybe filled with tasty candy and fun prizes?)
"I had heard Mr. Kurtz was in there. I had heard enough about it, too—God knows! Yet somehow it didn't bring any image with it - no more than if I had been told an angel or a fiend was in there. I believed it in the same way one of you might believe there are inhabitants in the planet Mars." (1.61)
Kurtz's name has literally no associations for Marlow—not even your standard Martian face. Does that mean he has no expectations about Kurtz's identity, either?
"The lustre of inquiring glance faded swiftly into vacant glassiness." (2.23)
Marlow associates death with emptiness since he describes the dead foreman's eyes as "vacant" as opposed to a once living "luster." Now just get a whole bunch of them and send them after an intrepid band of heroes, and you'll have a hit TV show!