If you've ever seen a James Cagney movie, or hell, any talkie that came out pre-Ben Hur, you've probably wondered, "Why the hell did people talk like that?" and "Why am I calling a movie a talkie?" One of those questions has a clear answer.
The people behind the YouTube channel BrainStuff - How Stuff Works put together an educational clip to diagnose the affected manner of speech prominent in American films of the '30s and '40s. It's not like your parents or grandparents talked like that, and it sure as hell doesn't sound like the way anybody talks anymore. Well, except when people incorrectly quote Jimmy Cagney and say, "You dirty rat, you killed my brother." He never actually said that.
The simple explanation: The Transatlantic accent, as this accent is called, was often taught in upper-class boarding schools in New England. But it was also hard to place geographically -- a great thing for geographically ambiguous talkies! -- and, moreover, the dropped R sounds and clipped T sounds came through clearer on radio and sound equipment back in the day, when, ya know, Beats by Dre was more like Phonographs by Bing Crosby.