I think there's a subtle suggestion that "she" is more of an active protagonist (since as Lewis says "she" is a subject pronoun), whereas "her" is more passive, possibly a person of interest for the actual protagonist or an important plot device (since "her" is a direct object word to which something does something). This analysis might not apply to these examples; it's just a thought. I haven't seen the movie or heard the song, but it would be an interesting comparison.

It's actually quite an interesting question, I'd never thought about it before. In fact, if a song was entitled either "She" or "Her" I wouldn't really distinguish in my head between them, except that I think "She" sounds a little bit more poetic and sexy, almost. I think that might just be because "she" is a nicer-sounding word than "her", but I wouldn't say that there's any difference in meaning - just as me is the object pronoun of I, her is the object pronoun of she.