The scene where Bastion is in the book store and soon the old guy goes to answer the phone and soon Bastion sees the book, he steals it and soon Bastion went, the old guy looks and smile. Basically, was the old guy only smiling because Bastion has left the store or did the old guy known that Bastion was going to steal the book?
I remember I used to think his smile had a more sinister meaning.. like he knew Bastian was going to find himself in some predicament he was going to have a hard time getting away from. But yeah, he knew Bastian was fulfilling his destiny by taking the book.
As to why he acts strangely, I'd say it's exactly for this. That is, make the viewer unsure about what his motives were and thus we'll keep wondering about it.
I don’t think his intention was for him to have Bastian take the book. In fact, he was trying to discourage Bastian from the book when he asked about it. When he smiled, I think he had taken a bit of a liking to Bastian after their brief conversation and then seeing him run off. Or, at least, that’s my understanding of the scene when you see the book store owner’s sudden look of surprise when he notices the book missing.
The book store man didn’t even introduce him to the book. Bastian asked about it. So he had to discuss it. I still don’t think Bastian was supposed to have the book. But I guess it all worked out in the end.
It's really interesting because the scene can be interpreted in such opposing ways and maybe this was intended by the filmmakers. Only the director and actor would probably ever know, which is why I wish there was an official commentary made. But then... it would ruin the mystery for us.
I still wish there was a commentary though, as there is so much more to talk about, but that's another story...
To me, at first it seemed obvious that the old guy smiled because he knew Bastian would take the book. But then the way he looks down and notices the book is missing, he seemed a bit surprised. Can't tell if deliberately ambiguous or just not the sharpest movie-making.