PLEASE pay attention to what your extras are doing. In fact, try not to cast extras. Try to cast ACTORS for all roles.
Nothing dummies down a piece of film more (and you’ll see this a lot on TV) than to have extras, say an angry mob, who are not angry, incapable of being angry and completely disinterested in what the whole scene was about anyway.
More and more I notice when the background is not supporting the foreground. So, watch for that guys. It can really make a huge difference in the quality of those types of scenes.