Just a little muscle research/design for the P.R.A. units. (Get ready, about to go off on a rant of boring crap.)

Ribcage sectioning allows for stretching/contracting of metal ribs, so as to create the illusion of breathing. Ribcage splits down middle, opening up to allow access to G.A.T. socket.
Midsection muscles swivel 'round to the front to pull upper torso forward (allowing for bowing and such) Spinal column also features servos which allow for more fluidic motion and realistic movement. Tails (which can be added for further customization) features same servos, though no muscles are required but can be added for stronger movement.
Eyes are 50x zoom capable cameras with 10 gigapixel output. Additional 'realistic' appearance is customizable, and is a unique fluid-filled film that reacts to static electricity to change the colour. Film is invisible to unit.
Ears are also customizable, and come in a wide array of styles. Muzzle is an extension of the skull, which can be added on for the appearance.
Skin includes a padding layer that acts as yet another illusion of realism. Fake intestines are added, which do not get in the way of synthetic muscles, and a fine film of lubricating gel is smeared on all metallic surfaces and muscles. Gel should be replaced once every six months, refer to your User Manual for further instruction.