Yes, I think Van Doren's smile was simply meant to seem phoney or forced most of the time, because the character was something of a phoney. On the other hand, I like his covert smile after he gets the answer wrong in his climactic scene; that one, at least, was genuine.

Actually, I think the moment at the book-signing party when he smiles winsomely and murmurs, "How did he know that?" was one of the most gripping moments in the flick.

I find Ralph Fiennes annoying so I'm not best person to ask. Everything about him including his smile irks me.

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@lgforce I didn't find his smile to be smug. To me it seemed forced, as if he was being congratulated for doing something that he didn't really do - which he was.
Stempel's tooth was at least three times as annoying. It grossed me out.
What ended up annoying me about van Doren was the countless number of times he (and his Dad) would recite some quote. Enough already!

It was like watching a department store mannequin.

I know what you mean, lgforce. I've just watched it again, and Fiennes' performance in general comes across to me as quite contrived, like he's trying too hard to create a persona for his character. His smile was certainly part of that.
To be honest, I'm not a big fan of Fiennes, because his acting always seems somewhat postured to me.
You might very well think that. I couldn't possibly comment.

the old man annoyed me.

I didn't notice his smile all that much because I was watching Paul Scofield smile instead and sighing over a 72 year old man, LOL!

I think that's exactly his charm.