Back on the plane (Spoiler)
I'm tempted to think that he was imagining the whole thing for his next book seeing that it began and ended on a plane. Although that would be a silly ending I suppose, so I doubt it. But we know he has a runaway imagination, as in his lecture he says he picks up small cues from observations and through a process of mnemonics his mind goes into overdrive creating connections and ultimately a story. On the plane at the beginning he was getting cues.
Maybe this kind of speculation was intended by the director.