The way people smile is different (but not too much, if it's a real smile). The way jaw, facial muscles, etc. work is the same. And the way the uniqueness of the character fades with emotional facial expression applied is actually right. That's how it works in real life, too.

However, as a separate point, custom morphs can affect how basic facial expressions work. That's the problem with morphs in general (or blend shapes or shape keys or whatever they call it) in any 3d package - they can conflict with each other.