Schweigen im Walde — silence in the forest. Duden: out of embarrassment or fear, nobody dares to say anything.
sich einbringen — to bring oneself in, to take part and make one’s contribution felt.
den Mund aufmachen — to open one’s mouth, to speak up.
seinen Senf dazugeben — to add one’s mustard. Duden: to give one’s opinion on everything, unasked.
einen langen Senf machen — to make a long mustard. Duden: to use unnecessarily many words.
Stille Wasser sind tief. — Still waters are deep. Duden: behind quiet people who do not express their feelings and views, there is more than one thinks.