You could type things in various situations instead as I recall.I just wish I knew how to shut her/him up. I can whisper a question to her/him. But then Siri blurts out the answer. There is no way to silence her/him without turning down the volume all the way and then turning it up afterwards. Kinda defeats the purpose.
A way to overcome this without leaping to a general universal setting is a "Siri, silence" command and a "Siri, speak" command.