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.

[Eomer of the Rohirrim]

Exactly. For Gandalf, physical violence is always the answer.

Seriously though Lathriel, they wouldn't have needed extra time to make Denethor how he should have been. They already took up a fair bit of time making him a complete jerk.

[Beanamir of Gondor]

This might sound naive, but in the chapter The Siege of Gondor in ROTK, which I read waaaaaaay before I ever saw the movie, I really saw Book!Denethor as a thoroughly unpleasant character. As soon as you meet him, you feel his distaste for Gandalf, his coldness towards Faramir ("Yes, I do wish that! For Boromir was loyal to me, and no wizard's pupil!"), and his utter obsession with Gondor. I only really started liking Book!Denethor AFTER he went nuts and silently sat at Faramir's bedside, white-faced.
So in one respect, I think that PJ messed up: Book!Denethor was unpleasant, all right, but essentially he really did love Faramir. That never, ever made it into Movie!Denethor's character, even if Gandalf said it out loud. That little putzy "Faramir?" at the end, right before he jumped off the Wall in flames, was nothing like what Book!Denethor said or did. I feel a little bit the same way about Movie!Denethor as I do about Movie!SiriusBlack.

As far as I'm concerned about Gandalf beating up the Steward, that was comic relief. Gimli wasn't there, so it was up to Gandalf.

[Eomer of the Rohirrim]

So much comic relief...