Grima and Saruman's Leechcraft

[haste]

I was watching TTT with one of my roomates last night and the part about Rohan had me wondering about a few things that I have never really grasp yet but have always wondered.

First, who was Grima before he became in league with Saruman? Was he just a citizen of Rohan who had climbed the ladder to become Theoden's advisor? Or was he specifically send by Saruman to begin with?

Second, and most importantly does Tolkien ever explain how Grima and Saruman actually take hold of Theoden's mind (in the movie, it doesn't seem so dramatic in the book)? Or is it just the councils of Grima that eventually lead to the withering of Theoden? I mean I'm not basing this off the movie because I know there are discrepencies. But I seem to remember always having that question on my mind and but never seem to remember it while I'm making a post here at the Downs. So if some of the Scholars could be so kind as to enlighten me a little, that would be great.