nice exercises, aren't they? Done them a lot.
IMHE people always get a bit confused about the uke and nage division. This is probably partly due to the way we train. One person holds, grabs or stabs and the other throws, locks or pins. Unfortunately the biggest misconception that is planted in the brain at that time is often the feeling and idea that one knows what is happening and going to happen and how the roles are divided (on before hand).
We try to teach people to realy have an open mind about the role of uke and nage (without turning it into a struggle or ego contest, which is difficult for somepeople). Just do the exercise you were shown b sensei and try to feel what role you get, let the union of partners determine the role, not your mind or whish.(Just remember that investigating the role you have is something totaly different from resisting or trying to block something!!) It is all about being in the moment, no remainder of the past, no looking forward to the future.
I found that this kind of approach not only improves your overall skil, but also forces people to let go of expectations about what is going to happen.