You might believe that motion picture photography captures reality and hence it is honest and true and real. You also believe that as a filmmaker it is wrong to try to modify that reality. So, you will pay more attention to ‘what to show’ when you make your films rather than ‘how to show’. If you think like this, you are a ‘Realist’ filmmaker.
However, if you believe in using cinema as magic, and using your expression to alter reality to create a different reality, you are an ‘Expressionist’ filmmaker. ‘How to show’ gives you greater satisfaction than ‘what to show’.
If you are shifting focus in the running shot, you are directing the audience’s attention. This is Expressionistic. However, if everything in your shot is in focus and you leave it to the audience to focus on what they want, this is Realistic. The ‘unreal’ sets of a Sanjay Bhansali film can be defended by the concept of expressionism – it is the director’s wish to supersede reality with his own way of looking and showing. The complete lack of background score in various movies can be defended by the concept of realism – we do not have background score in real life!
It is difficult to find examples of films relying totally on either of the philosophies of filming. Generally, the best of cinema uses the best of both.