When I first did this it was with my brother in the car, so it didn't matter that much how I looked. I'm at the point now that I can just pretty much do mouth it once and it works so it isn't much of a problem in public. It probably doesn't look too natural, but I guess repeating a word 5 times or blocking for 5 seconds isn't either. People might try to complete the sentence for you which doesn't help you improve. If it's someone you know, maybe ask for them to not do that.