I watch a lot of old movies and I've noticed that the American actors of the and 1930s and 1940s unusually spoke in a quasi-generic-posh-British accent. It seems exaggerated and I imagine it was not common off the stage and screen. What was that and did real people actually speak that way? Was it just an accent or was it a dialect?
As mentioned in a previous answer to one of your questions, this is called Mid-Atlantic English and was commonly used in American films of the 1930s and 40s.
Wikipedia gives the following reasons that someone would use the accent:
So essentially, this type of speech was never common and was only natural in the case of ex-pats.
Some examples of 'Mid-Atlantic' speakers:
Speech was a lot more varied than what was shown in the movies of the thirties and forties, particularly since those films romanticized the lives of the rich. The average citizen on the street sounded quite a bit different. My grandparents were born in 1919 and 1925, so they would have been young in the mid forties. They were from Brooklyn and came from simple backgrounds (my great grandfather was a fireman, so he had about as much in common with some highborn socialite as a rock has with a piece of cheese.) My grandparents actually sounded a lot more like Bugs Bunny than any character in the Philadelphia Story; they had every feature of Bugs's speech except pronouncing ur sounds like oi, so that "turkey" became "toikey". (Another good example would be Mel Brooks, who is still alive and was only a little younger than my grandmother; he grew up on the Lower East Side of Manhattan, which during the war and the Depression was heavily populated by Eastern European Jews and their American born children: his speech and his comedy are to this day peppered with idoms and expressions from Yiddish.)
People from other areas, the average person, sounded very similar to the way they do today. If you look up a man named Babe Heffron on YouTube, he was a soldier in a famous company during WWWII (the same one that invaded Normandy as paratroopers and marched on to free Dachau.) Babe Heffron sounds like a typical guy from Philadelphia and his speech is not much different from the way his grandsons (men in their twenties now) talk today. Louisana's accent had a stronger French component than it does in the present owing to a heavier concentration of Cajun French speakers. Black people's speech had a more southern flavor right on up to the 1950s because they were in the middle of a huge migration out of the South and most still had contact with it. The only big difference I can think of offhand might be the Boston accent, in that it was a lot thicker in the forties and thirties mainly becaue it had been isolated for so long, there were not as many people moving in from other areas of the nation (they used to call a drinking fountain a bubbler, but few people I know do this any more, and some rhoticity has come in, though not total.)
There are still examples of people with mid-Atlantic accents. The late William F. Buckley and his nemesis, the late Gore Vidal, are pretty good examples. I've heard WFB's son Christopher Buckley talk, and he also exhibits this accent. The actor Kelsey Grammar also has the accent, it wasn't just put on for his Frasier character.