Let me throw in a few words of defense for Michael Sheen.
Yes, his laugh at Nessie is childish and ludicrous. Michael was hamming it way up, he was having fun with the part. However many shots of this they took, though, this is the one they kept. I think there were a few reasons for this, beyond the fact that this was Michael's role and they gave him some deference to how he wanted to play it.
* In my opinion, this bizarre laugh is a clue that things are not going to proceed quite according to the text.
* Michael is emoting. Suzan's point is spot on about him making his intentions almost embarrasingly clear. That was the right way to go though, in my opinion. He's doing this for clarity, as they decided not to include some other things from the text itself that makes it clear he is constantly lying. I'm thinking of when he turns around and talks about the need for secrecy and Caius is there smirking. Yeah, it's hammy but it leaves no doubt whatsoever what their true intentions are.
* Michael is camping it up, playing the obvious role of the villain. He wants you to want him dead. I love the look on his face when Bella is trying to pull his head off.
Is Aro slightly insane? Well, he killed his own sister so he isn't quite right. He is definitely evil, and I think Michael had to make a decision how to go with this, playing a dark controlled evil or camp it up and play an entertaining and obvious evil.
I think he made the right choice, but there is no doubt that laugh is WAY over the top. If I had to make a guess, I think he really did it because he wanted to show a couple of different facets of Aro. He really was extraordinarily curious about finding new things in the world, and discovering that half-breeds were actually possible and seeing a delightful one right in front of him tickled him pink. Book Aro wouldn't show it that way, but Michael decided he wanted to. And, this also shows how evil he is, because the one who is giving him such joy and wonder standing in front of him is the child he has every intention of killing, along with anyone else who gets in his way.
I mean, can you believe it? This precious child shows Aro her thoughts by laying her hand on his cheek. Aro is able to view Nessie, this extraordinary talented beautiful unique child, with wonder and delight, while simultaneously she means absolutely nothing to him. He wasn't moved one scintilla, when he should have melted in a gooey pile of emotion. Aro is disturbingly wacked.
Was it all too much? Actually, I kind of liked it, but I can definitely see where others would have just been annoyed with him.
"It will take an amazing amount of control,” she mused. “More even than Carlisle has. He may be just strong enough…the only thing he’s not strong enough to do is stay away from her. That’s a lost cause.”