[nkrisc]

That’s a good point, in the movie they only show the already “broken” Denethor.

[Ichthypresbyter]

In the extended edition of Two Towers they show him in a flashback of Gondor retaking Osgiliath.

It's not a particularly flattering portrayal- the military success is shown as belonging to Boromir more than Denethor- but at least it shows him sane.