Denethor's madness.

[Lathriel]

I also find that Denethor's character is static however it doesn't bother me too much because there is too little time for him to develop much. I mean the movie is already 4 hours long and then to add character development for Denethor...it would simply bring the movie down. Although Denethor is an important character he is also a very minor one. Even Faramir has a bigger part to play then Denethor does.
As for Gandalf wacking his stick at him, why not? Denethor was getting annoying anyways, and he had to shut up.