And yeah, I'm a HUGE Heinlein fan. He might be my all-time favorite author.
I grok it! I've read most of his novels and short stories; he and Asimov are in my top five.
When it comes to casting SiaSL I would go with "unknowns"; the characterizations are so delineated that known talent would have too much baggage to make the performances believable. (The film on which I am currently working has an actor who would make a great Jubal Harshaw.)
I think that an "easier" Heinlein novel to turn into a film - and more fun (meaning box-office success) - would be "The Moon Is A Harsh Mistress." Another cool adaptation would be "The Cat Who Walks Through Walls," or perhaps even "The Number of the Beast."