As PC said, take a personal stereo so you can block out the noise and pretend you're not there. Dark glasses or one of those eye-cover things they give you to help you sleep on aeroplanes helps too.
My dentist rubbed anaesthetic gel on my gum to num it, then gave me a little injection (which I couldn't feel cos of the gel) and then when the little injection took effect, gave me the big injection to really numb it properly. Didn't feel a thing.
He/she should arrange a signal with you (waving hand or whatever) which means "stop, it hurts" or "hang on, I'm really uncomfortable".
The main problem with root canals is lying there with your mouth open for a long time. But it is just uncomfortable, no more.
Get co-codamol tablets (ask at Boots for their own brand) and take them before the injection wears off - its liable to hurt a bit for a day or so afterwards (nothing like it hurts now though).
And buy yourself some really nice soup so you don't starve while your mouth is all numb!
Veteran of many root canal treatments here!