Hitler's home movies
This is most interesting to me, because I'm a great fan of silent movies, and I'd love to see this new technology applied to silent films, so we can learn what the actors were actually saying. Intertitles can never give the whole story, you tend to get just the essentials. Plus, there are all those stories about what was REALLY going on when there was no sound recording equipment to catch what the actors were saying. I've heard that there was a lot of filthy language being said, while the action was all of love and romance - that scene in 'Singin' in the Rain' where Don Lockwood and Lena Lamont insult each other while acting out a passionate love scene isn't all fiction.
This wasn't the case with Fritz Lang's films, though. He wouldn't let anyone improvise anything, not a word or a movement. That stereotype of the movie director with the monocle and the jodhpurs, barking orders through a megaphone? Lang INVENTED that character - he was an absolute dictator and a thoroughgoing bastard. I can lipread a bit, and where there are dialogue cards, I can see that the actors are saying the exact same words. But a lot of stuff isn't titled, so it would be fun to know what else the characters were saying. I suspect it was all in character and scripted - Lang would never let anyone just make stuff up.