uhh.. denethor did look into the palantir.. that's why he's so hopeless and keep thinking it's the end of minas tirith.. and wants to kill himself... and that's why he wants the ring.. sauron was showing him things through the stone.. it was in the book... it was mentioned he kept the stone high up in his tower... when faramir's injured body was brought to him, he went up to the tower and all the guards saw a great flash... when he came down he looked old and weary... it was also said that before that incident... the guards noticed he went up to the tower quite often.... and gandalf said he did that to gaze into the stone so he DID look into the palantir and he was so crazy because of the enemy's influence.. whereas in the film he was potrayed as a guy that's already crazy... in fact... i didn't like it when he was actually killed by gandalf in the film... but still... IT'S A GREAT MOVIE !!
