What does Lord Denethor try to do when his son Faramir returns to the city uncoscious?

What does Lord Denethor try to do when his son Faramir returns to the city unconscious? He tries to burn Faramir on a pyre.

In The Return of the King, Faramir is gravely wounded while defending Osgiliath and is carried back to Minas Tirith. Denethor assumes that his son is dying and, overwhelmed by grief and despair, orders a funeral pyre prepared for both of them.

Denethor's decision is tied to his collapsing hope for Gondor's survival. He has been psychologically affected by using the palantír and by the death of his elder son, Boromir. Believing defeat is unavoidable, he chooses suicide and intends for Faramir to die with him rather than face the coming destruction.

Pippin alerts Gandalf, who reaches the pyre in time. Gandalf and the guards rescue Faramir from the flames, but Denethor lies down on the pyre and burns himself alive. Aragorn later heals Faramir in the Houses of Healing. The question is specifically about the film's plot, though the same major event occurs in Tolkien's novel.