If I have to share space with a stranger for a long enough time (say, on a plane or train, or in a café), and he/she looks like someone I could strike a conversation with, and the person stops reading, I would probably use the book as a conversation starter, and have done so before.

In most cases I wouldn't. It's rather rude to interrupt someone who's reading where I live, all the more a stranger. A lot of people read in public to isolate themselves from people around so you're invading their privacy if you interrupt, and while most people will notice what others around are doing while commuting etc., it becomes intrusive if you go and flat out mention to the person that you've done so. It's quite bit the same as staring. A lot of people won't mind and be friendly if you approach them, but a lot of people will find that impolite/intrusive.

As far as I'm concerned, if someone approach me about what I read, it's also very situational. Sometimes I won't mind, sometimes I will.