One of the interesting facets of the story of The Man of La Mancha is why people get drawn to DQ. You can also see this theme in several of the works of Chesterton, perhaps most notably in Manalive. The idea of a man whose insanity takes the form of actually achieving the childlike goodness that we all approve of in the abstract, but don't have the energy to actually do, and why people are drawn to this. (One might say that the original would be the story of Jesus Christ, but while Jesus embodied that goodness, he didn't do it in a childlike way. Jesus was good, but also quite aware of the evils of the world and quite shrewd.)
In The Man of La Mancha, the character that we can relate to who is drawn to DQ is that of Aldonza, a prostitute. There is also Sancho Panza, but he's something of a simpleton. It would be very interesting to amalgamate these characters, and reverse the idea of the weak being drawn to the madman. Instead, someone powerful like an extremely rich CEO, or better yet someone like an assassin, a drug lord, or someone else who's very powerful and very deadly and up till he met the madman, very bad.
It would be good to have someone who knew him from his former life, before he became the madman's lackey, to ask him why. He could explain something like, "I was practical for the first thirty years of my life. It made me very wealthy and very powerful, but it never made me nearly so good as when I followed this madman. I thought that I used to be happy, but now I know what happiness is. I thought that I used to laugh, but now I know what laughter is. Even if I'll never be a saint myself, I'd rather smile with the saints than smirk with the sinners. Just a moment of real happiness is worth several lifetimes of fun. More than that, I can't explain. The world we lived in is like being inside of a balloon. It's very complete, but it's very small. If you've never been outside, I can't tell you about it, other than to say that the world is a lot bigger than it looks when you're inside the balloon we called being realistic. I wish you luck in finding it. And now, if you don't want me to become practical again, get the hell out of my way." (This should be said in some manner where the friend is blocking the way, and the protagonist is obviously threatening to break him.)