Now I go to frame 10, and apply a partial pose that I've previously saved to the library. This is devised so that it will only pose his right forearm, and it does.

BUT - and this is what digitani is asking about - it sets keyframes on all of Simon's actors, even the ones that were not included in the pose file. This means that they're 'pinned' at their current value, which spoils any interpolation that may be going on. A pain in the arse for an animator.