I can't pretend that I wasn't a little nervous - most of my role preparation has been on my own, the flu etc meant I hadn't really had time to consolidate, and much of the production I'd just seen from the sidelines. Of course, I made careful notes of what was happening where, but such things tend to escape from the poor singer-brain in such situations... you have to remember where your cues are, what the words you're singing are, who you're meant to be singing them to, and what you're meant to be up to at the time. The latter is not helped by the fact that switching from the audience point of view to that of the performer often completely snarls up your sense of left and right (whilst sinking to my knees at one point, in order to curl up on the floor, I had to think what the scene had looked like from the auditorium and which side her head had ended up on, then mentally turn the picture around and act - all whilst in the process of actually doing the action!).
I got through, luckily, without any major snarl-ups (and immensely grateful for the work of the prompter!) - I would have hated to mess up anyone's performance, as the whole idea of stepping in to rehearsals like this is so that the process of creating the production can continue; so long as you don't cause problems for anyone else, small mistakes are overlooked.
However, whilst it wasn't *about* me as a cover, it was enormously useful to have the opportunity to do a rehearsal like this. Weak points in my learning were shown up, so that I can pay particular attention to those when practising, and the sheer kinaesthetic experience of actually playing the part on stage means that should I ever need to step in to an actual performance, I'd be a lot happier to do so. Hooray!