Ghostbusters said:

One thing that surprised me about the prequels was how bad Ian McDiarmid performance was in in AOTC. I remember the first time I saw him in that I was like, that's it, what happened to the snaky performance he had in TPM? So much character of him in that movie compared to AOTC. He just seemed to be standing around stiff saying a few lines. In ROTS he was better, but still was missing something.

I don't care what people say about Liam Neison, I thought he did a great job in the TPM and comes off as one of the few people to pull off a believable and convincing performance in the prequels.

Interesting observation - in TPM, he seemed to play the role of a snaky, greedy politician trying to conceal his ulterior motives.

A role that made sense in the context of the movie, maybe, but considering the unreveal that he's actually Sidious, the fact that he couldn't "act" well enough in real life to conceal his obvious personal excitement at being elected. And then of course his "oooh, and of course I promise I'm going to help you with that crisis of yours" face when Padme looks unimpressed and almost disapproving.

In AOTC, whenever he expressed something, you could feel the phoniness in his face and voice, obviously but subtly. Which was nice.

I'd say his performance there was on par with Christopher Lee's. Not a complete monotone cardboard cutout, and "convincing" and somewhat charismatic in their own way, but not remarkably alive and memorable, either.