Denethor's madness.

[Thinlómien]

Originally Posted by Boromir88

Or, whether Jackson just showed Denethor being a crazy guy from the very beginning.

He did so.
Probably PJ thought also about the loss of Boromir, if you remember the scene in RotK EE where Denethor looks at Faramir and sees Boromir running to him (Denethor) and says "ohh, my son."

[Essex]

In the commentaries, Ian Noble mentions that Denethor was affected by the Palantir, and I know in press conferences he held, he was sad that jackson was not able to fit in denethor with the palantir. maybe just a view of the palantir burning in the pyre as we get in the book would do.

but movie wise (without knowing the story) then yes, he was driven mad by the loss of his son. I don't think he was 'crazy' in the film though until he sees the hordes of orcs outside minas tirith. that (movie wise) tips him over the edge.

[Tuor of Gondolin]

Aside from Gandalf's ridiculuous phyically assaulting Denethor,
the worst bit of movie Denethor was the lack of any
character development or even character collapse. Even in the
extended dvd bit when his boys take back Osgiliath Denethor
is portrayed as a thoroughly unpleasant person with no
discernable plusses. There was no character arch,
which even movie Faramir had, to some extent.

[Essex]

Originally Posted by Tuor of Gondolin

Aside from Gandalf's ridiculuous phyically assaulting Denethor,
the worst bit of movie Denethor was the lack of any
character development or even character collapse.

Ok, I understand your point (in bold), but I think Jackson uses his creative licence from this part of the text.

But Gandalf sprang up the steps, and the men fell back from him and covered their eyes; for his coming was like the incoming of a white light into a dark place, and he came with great anger. He lifted up his hand, and in the very stroke, the sword of Denethor flew up and left his grasp and fell behind him in the shadows of the house; and Denethor stepped backward before Gandalf as one amazed.

[Eomer of the Rohirrim]

I think you are right Essex, but (given the opportunity) I would have told Jackson that it doesn't actually say that Gandalf bashed him on the head!

[Tuor of Gondolin]

Good point by Essex above about Gandalf using his powers
when applied to the funeral pyre, when it was clearly necessary to
exert himself to save Faramir. The problem I have with PJ's version is
1) it's used earlier without the immediate threat of Faramir being killed
2) at the funeral pyre PJ misportrays it again as just slugging
away with his staff (as quoted from the book it would have worked better).
A wizard's staff should be used basically as a conduit of power, not as
a baseball bat or club.

The hidden power in Gandalf is all the more striking when sparingly
and correctly portrayed (as PJ did in Moria).

[Boromir88]

I do agree that the beating up of the Steward was another thing that Jackson overused. The way I see it when Gandalf storms in to save Faramir, and Shadowfax rears up and kicks it, it is more Gandalf that kills Denethor, and not Denethor. I didn't see any point in that part, basically had Gandalf give Denny a little nudge on his way out.

A wizard's staff should be used basically as a conduit of power, not as
a baseball bat or club.

I would say that's an accurate portrayal of the events before rescuing Faramir. Essex's quote, Gandalf strikes him to get him to back down, to sort of scare him. Where Jackson just had Gandalf whack Denethor until he was KO'ed. It wasn't enough to hit him in the face, but he had to jab it in the gut, then get a cheap knock over Denethor's head to make sure he was out cold. I'm surprised Gandalf didn't kick him afterwards.

I think that scene would have worked well, since Denethor is running around yelling at everyone to "flee," and Gandalf sort of conks him to either get him back to his senses or shut up. But, PJ couldn't have stopped it at that, there was really no need to beat the guy until he was unconscious.