I personally rarely make conversation with people by me on a plane (basically the same, right?). I usually like my personal space, and usually like to sleep or read. I've had brief conversation with a few people on trips, but rarely over 10 minutes or so.
As far as a girl falling asleep on my shoulder on a trip like that? Well, if she had caught me on the rare occasion that I was making general conversation as we took our seats, if I thought she was cool and/or cute, then I wouldn't mind. I'd probably let her either way, but it'd just make me more willing if she were.
