Denethor's madness.

[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.