well I may be a little late in this reply.....I've been snowboarding
anyways when training at my dojo I haven't had a chance to be a sempai as I am fairly new but as kohai I keep silent, some sempai will talk and explain the movements. I have one sempai whom I really enjoy trianing with, she is very pedantic and stresses all the tiny little details of movements until I get it correct, she is very verbal and I enjoy her explinations as I manage to soak up and remember everything she says. Every sempai I train with has something different to add, their own interpretation of the movement and I never disregard any of them even if I have been told something different from a sempai at a higher level, instead I try to incorporate everyones input into the movement, using what works and discarding what doesn't. I enjoy training very much but I don't think a sempai should be completely mute, little pointers here and there serve incredibly well and add to the overall learning of everyone involved.
just my scattered two cents