Stan Winston Schools has released other behind the scenes footage of the T-600 puppet from Terminator Salvation, but this gives a clearer sense of how the animatronics evolved and how important the materials are in any puppet. If it's not lightweight, but with the stability and feel of whatever it's representing, it can't be convincing.
Also cool: in this footage, we get to hear John Rosengrant, a co-founder of Legacy Effects and a longtime Stan Winston Studio supervisor, talk about the importance of choreography and how puppet footage is integrated into a final cut. He talks about how even the most perfectly timed scenes will only show a few frames at a time of the puppet, because its realism doesn't hold up in the long term. Working on something like this would pretty much be the coolest job ever, wouldn't you say?