It might be the acting method. Take this with a grain of salt as I don't k mnow this that well, but: There was a new acting method invented that started catching on around that time. It's rather awkwardly called "method acting" and it basically involved pretending you are the character, really immersing yourself in their thoughts and their life. Some actors take it a bit far, basically being their character even at home while working on a movie, but, man, it made a difference in how the portrayals felt.
I think that's the big change, I'd love to ask someone more knowledgeable to be sure.
I still like the older ones too, but they do feel rather different.
It might be the acting method. Take this with a grain of salt as I don't k mnow this that well, but