well. It would be more interesting if he built the flying robot and Yinsen had to deal with that. Because it's not like Tony would magically stop trying to solve problems with engineering. But Yinsen might be the only human on the planet who'd be able to talk to him about it.

re: the cave - I hadn't realized it until now. But he doesn't try to crack one joke. it's really unusual in the scope of the film, and I think that's the real Tony Stark. We get glimpses of him after that (and when he mentions his dad to Christine) but really he keeps the damn game face on nearly all the time, doesn't he? He even does it with his robots, to a certain extent, when no one is looking.