Usage patterns
〈人〉は寡黙になるfor someone to become taciturn/quiet
My father became taciturn after that accident.
〈人〉は寡黙を貫くfor someone to remain silent throughout, maintaining a reserved stance
He remained resolutely silent throughout the interview.
寡黙を装うto pretend to be taciturn/quiet
Although he actually wanted to talk, he was pretending to be taciturn.