[@Birdboy]

Your challenge wouls be you'd need to line up altentive forms of expression and communication beyond words. That's harder to convey in posts, doable but harder. 

And its usually triggered by stress, pressure or such things, so you'd have to narrow down what the trigger / release is for the mutism ahead of time. 

Also narrow down, the how, when and the mechanics of the character ahead of time, all the little things first. Alot of prior prep and planning..