I get the impression he really enjoyed the part, and that that mischievous aloof sense of evil was part of him somehow. I wonder if some things such as the snuffing of candles with fingers and tongue, and the sidestepping of certain God-centric terminology in casual conversation (even with Maggie! "...before... your Christ was born..." !) were improvisations from him... does anyone know?
Still, as great as he was, I don't think any actor can pull a poor script out of the fire, and though he made 1968 much better than it would have been, to me he's much more enjoyable as the Nicholas Blair wanna-be Evan Hanley.