Do you remember seeing well rendered CGI characters, whose acting was bad? Not that the character is drawn badly or doesn't quite look real - that's not the issue here - but that his or her acting comes across as bad?
I was thinking about the orcs and goblins in the Hobbit, and they are done very very well. It helps that they don't have hair or clothes. But not only do they look good, but their acting is good. Likewise Gollum looked basically real to me in the LOTR trilogy, and his 'acting' was fine.
Now think about the scene in Up where the little boy scout first comes to the grumpy old man's door. The boy is reading something about helping the man across the street or whatever. His delivery is quite flat, but it is supposed to be. You know the boy is of course a a cartoon. This one doesn't count, since the 'acting' is supposed to be bad.
A better example is Tron 2. I assume that the young Jeff Bridges in Tron 2 is computer generated. He's done pretty well, but the 'acting' seems kind of stiff. The guy is tense and angry all the time . . . you don't see a lot of range of emotion and the way the film is colored the characters are washed out a bit. Not bad acting, but not great either.
Can you think of examples . . . where the GCI is done so well the character (human or otherwise) looks real, but the acting is unintentionally not very good - stiff or over-the-top or maudlin or somehow just plain wrong?