I just saw (on DVR) the 1931 version of The Maltese Falcon. It's not very good, but fascinating to compare to the classic remake. Much worse acting, pacing, etc., but the cast of characters is exactly the same, and each 1941 performance was so indelible that it's startling to see someone else trying to do what's recognizably, at a general level, the same thing.
Weirdest difference: the Sam Spade character is grinning broadly about 95% of the time. (This was not exactly Bogart's approach - even his smiles are grim.) Best difference: it's pre-Code, so the film is far more candid and explicit re. Spade and Miss Wonderly.