I kept quiet when it was an elderly lady at my door that I knew had been a JW all her life. I have also passed street witnesses with mixed results.
I passed one at the supermarket and simply told him, in his language: "Your religion is false". That was as much as I wanted to tell him. Another one was in a wheelchair and my pity for him kept me from saying much.
I have yet to be aggressive (in a polite manner) with anyone. I keep telling myself that I should say something of substance but I'm a non-confrontational person at heart. Damn!