Sometimes the TV or movie version of a novel lets you down. Often it does. But in the case of Dexter, I found the novel too dark and expected to find the TV version even darker. But because of the talent of Michael Hall, I am able to watch it.
He gives a humanity to Dexter I was never able to find in the book. Although he may tell the viewer, he can't love, you can see that he can indeed experience love when he looks at his sister, or his girlfriend's kids or perhaps even his girlfriend. The message in his voiceovers are softened by the glint in his eyes, the delicacy in his touch.
Can you think of other examples of actors that made a character from a novel more palatable or lovable or real. Usually it goes the other way. Although Anthony Hopkins created a memorable Hannibal Lector it wasn't bu softening him, but by creating a character you couldn't take your eyes off of. In a TV series, that probably wouldn't work.