HAA is the only person that could have played Nicholas Blair. The chemestry was great with a lot of the actors it seamed so real. GH, MW, LP did put out fireworks when working with HAA. That is the only thing that I liked about 1968 story line was not the story, but the fireworks with all the actors. Besides the flubbed lines, you can see that the actors really got along.
Yes, when you watch Humbert in a scene, notice how "connected" he is with the other actors. By this I mean, he doesn't stay in his own space, isolated like some actors do, he has great eye contact, and usually he will touch the other actor in some way, like putting a hand on their shoulder, or with the ladies, taking their hand in his, that type of thing. I think this technique makes him a more exciting actor to watch, and gives him that, shall I say, sensuality. Also of course he has an amazing powerful voice (as does Jonathan Frid).