Solojones reported in another thread that Hanks passed on the project without even reading the script. I can see why. He's already been called "the new Jimmy Stewart" for a lot of years now, it would have been too obvious a choice, though earlier in the thread I believe I thought that's exactly who Spielberg would cast. Hanks could have given the greatest performance in the world and yet people would have said, "Hanks for years we've been saying you're our era's Jimmy Stewart, well we've seen your performance in Harvey and Tom, you're no Jimmy Stewart." He would inevitably suffer by comparison. Downey on the hand, assuming the update on the script is interesting and fresh, seems to me an inspired choice. I think it's even possible that this piece of casting is what may have given Spielberg the confidence to go through with this. Who could do a better job?