First of all I wanted to say that Sean Penn's perfomance in this movie is brilliant, befitting the king. I could not find any weak points in his acting (maybe except for the last scene of the movie but once you see it, you'll understand). I was not looking at Sean Penn on the screen - it was Willie Stark, with all of his cold calculation, love of power, readiness to destroy opponents using blackmail and other "dark" means. Still, Willie Stark had the magnetic and powerful personality and was able to give great speaches which won him the people's hearts and votes. He was a brilliant if controversial and corrupted-by-power politician.
In particular I love one scene from this movie. During one of his numerous speaches, Willie Stark says: "Nobody ever helped a hick but a hick himself." Catchy but straight talking.
(more quotes here but this movie is a must)
source of the poster and the photo