The theatrical adaptation of LoTR portrays the undead from Dunharrow considerably differently than the books (see this question, this answer, and wikipedia). It also changes the role they play from support to essentially allowing the battle of Minas Tirith to be won, rather than lost.
In the movie, they are shown as somewhat hokey "easy button" to resolve the battle by allowing them to come in and trample/kill all forces of Mordor.
According to a magazine article, Peter Jackson hated the Dead Men; he thought it was too unbelievable. He kept it in the script because he did not wish to disappoint diehard fans of the books
This has always been my least favorite part of the Lord of the Rings theatrical trilogy. It just feels lame to me. But I didn't realize Peter Jackson also disliked them.. but then made them even less believable than those in the books.
As a result, I am curious what reasons exist for the men of Dunharrow were portrayed in this way. Note this thread has a fair bit of anecdotal thoughts but I am hoping for a bit more comprehensive answer.