I think that sometimes, when Steph and Cass are out on patrol and it's just them. Steph will stop talking and just let Cass read her body language. Not for any real reason other then the fact she knows it's easier for Cass to just read body language then to take the effort to translate movement into words.
I bet this also comes in handy when the two need to make a plan in silence, Cass can read Steph to get the plan and nod or shake her head.
I can just imagine them playing 20 questions or some other "yes or no" game in the backseat of the batmobile, and Bruce looks into the back to just see the two sitting in silence staring at each other with Cass nodding or shaking her Head occasionally.
Steph doesn't know but Cass can also read the fact that Steph sometimes thinks about kissing her. It totally only happens rarely...right?