Talk, moan, move... do something to let me know you are alive and present. One shouldn't have to hold a mirror up to their partner to see if they are still alive. If that is the case, then neither party should have gotten nekkid to begin with.
Everyone is different so whatever one found pleasurable/arousing won't transfer to the next. A couple should feel unafraid to communicate in bed. Although MM does have a good point of view. I, personally, would want to be told pointblank, "This just isn't working, would you mind if we did that instead?" I mean, how am I supposed to know if that other person doesn't share that with me?