Haha yes, "I need you to evacuate" is certainly no good -- but "evacuate; this building is burning and you will be trapped soon otherwise" is much better than "evacuate". People who grow used to urgent-sounding requests are likely to blow them off.

I once got fired for not doing something the way the boss wanted, because at my previous job we did it the opposite way. He had explained that he wanted me to do it, but didn't say why, so I didn't remember. I too find that when I ask someone to do something, they're much more likely to do it right if I explain why, not just how. (Getting the method wrong is different from ignoring the request, but they both spring from the same underlying phenomenon -- we are likely to discard information if we don't know why to heed it.)