Fast forward to the scene where Westly is wrestling with the ROUS in the fire swamp. When the two are rolling on the ground, at one point Westly punches the rat and it rolls away. Right when the rat stops rolling it reaches up to it's head and "pulls" it back on, as if it were loose or something (I'm assuming the person inside could feel the head slipping)

My dad pointed this out to me when I was like 4 b/c my brother and I were scared of the rat and it made the scene less scary for us. It was "proof" that the rat was someone in a costume.