I noticed a marked improvement in Arnold Schwarzenneger's ability to move like a cyborg (ie. move his head slowly, turn before moving...) in Terminator 2 as opposed to The Terminator (1). Likewise, I noticed a reduction in his abilities in Terminator 3 as opposed to Terminator 2. Anyone know why this is? Was it Cameron's absence?
#1 => #2: Multitude of simultaneous reasons: older, more acting experience, better English skills, more "human" role (T1 he was supposed to be a machine killer, T2 he was supposed to become gradually more human-like).
#2 => #3: The technical term for his performance is "Phoning it in".
In the Ultimate Edition DVD the director's commentary (Jim Cameron) states that
"I've never seen an actor to take physical direction like Arnold".
Is it possible that Schwarzenegger and Cameron gelled to the point that it would produce the kind of results that you're talking about? Mostow was the director for the third film which might explain any differential from II to III.
Witness his performance in True Lies as well for further evidence of the partnership between Cameron and Schwarzenegger...