When do you need a model release

When do you need a model release? That's the latest question I received from one of our listeners: "What guidance can you give us regarding releases? Both for people (for instance, if they are potentially identifiable) and locations (for instance, trademarked property, federal property)? I seem to recall this gets complicated. What if it’s a crowd such as demonstrators in DC? Assuming a release is required, as a practical matter do you obtain it during the shoot?"