The scene: I'm in the window seat.
Describe your fellow traveller: He's slight, neatly turned out. Thin moustache. Looks like an anxious substitute Chemistry teacher, down to the pens in his top shirt pocket
The review: As he takes his place in the middle seat, he doesn't say hi, despite my half turn and eye contact with raised eyebrows that signal I'm open to remedial levels of introductory engagement. He settles in quickly and immediately dominates the arm rest, ignoring the common politeness of the 'one person's elbow goes to the front, the other person's elbow goes to the back' unspoken rule. His arm squats along the entire length of the arm rest for most of the flight, despite occasional sorties by my arm. I can't get a toehold on any of the shared territory. After dinner, though, he is admittedly helpful with some basic tray-passing and trash-relaying, so he's obviously not completely self-centred. He doesn't read or watch a movie. In his waking hours, he just regards the back of the seat in front. Hard to peg. I'm putting him down for slightly nervous, lightly selfish but not a full-on shared-space bully.
Verdict: A mostly benign mixed bag.