Ultimately, I always like to let the actors work within the character and bring their own phrasing to the lines if they want to do that. Some actors don’t like to, and that’s fine as well. The actor isn’t a writer, and they don’t necessarily know the context of the scene in the film as a whole, so it’s not their job to create, it’s their job to inhabit. But, I always like having another voice in the process, saying if a scene doesn’t make sense or if a line doesn’t feel right, and working to make it feel better.
So, Altman’s process has its merits, but so does the more traditional way. It’s awesome to have a film as a happening, as something that feels real in the moment of film, but ultimately what you need is for it to feel real when the viewer’s watching it. That’s the take away, and if you succeed it doing that, it doesn’t really matter how the specific lines came to be.