In the case of larger stars (I mean prominent film stars, not Raymond Burr), I think it comes from their knowledge they are famous, that everything they do is scrutinized by the press. It unfortunately spills into their work, where they say "Hey, look at me, I'm playing a psycho killer this time. Remember me at Oscar time." There's also the money that the biggest stars make. When so much is riding on a "name" to sell a film, that "name" becomes a minor character in the film. "Hey, let's go see Jennifer Aniston play a nuclear physicist." For the vast majority of actors who are not well-known, I think they are still willing to "immerse" themselves, mostly to impress the people who might see them and help them continue to get hired. It's a cliché when these actors are interviewed and they go on and on about "just loving the work", but I'm sure there are actors who genuinely want to do good work as opposed to doing fame-seeking work that gets them into that rich-and-famous tier (who can then do lazy work and still sell tickets). It's likely the producers, writers and directors (jaded bunch they are ) who shape the work of many of their actors by having them behave in less-than-realistic ways onscreen. A director who openly looks down on his material will make a product as bad as he thinks it is. Writers who try too hard to be clever (to show how "talented" they are) end up producing material that is difficult for the viewer to understand and "believe". Producers who are too wrapped up in the marketability and profitability of the project (with so much money on the line) to notice any of this. I think a segment of those in charge also want to show how "above it all" they are when producing films for the mass audience (The Great Unwashed in flyover country) and I don't think they even realize that air of detachment is interjected. And an even smaller segment doesn't care if it is.