I noticed upon my first reading of the novel that Blatty really seems to dwell on people leaving. Perhaps a dozen times in the book he describes someone watching as someone else goes to their car and drives off, or walks around the corner and disappears, or what have you. Probably the most potent example of this is in Father Karras's dream when his mother descends back into the subway. What do people make of this? It seems like an odd motif, one that doesn't come across in the movie.
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(I suppose in the movie, a similar/equivalent visual motif are stairs.)