Not much to add really, all I remember about going behind the scenes in Space Mountain was standing behind a curtain and being able to see the ride (although it was very dark) - seeing how fast it went was very offputting for me!! But interesting since it's an indoor ride so you wouldn't ever be able to watch it otherwise.
It probably didn't help that we did the tour after we'd already been there for a couple of days, so our feet were already tired, but we really felt exhausted afterwards! We had a cast member called Bala and he really rushed around! We collapsed in seats in the Discovery Arcade afterwards! It was interesting though, when it first started I wondered if it was going to be really basic and we were already going to know a lot of stuff since we'd been before, but Bala told us a lot we didn't know.
There are 2 tours a day, and they're done in different languages - obviously the most common are French and English but there's also Spanish, German, Dutch and Italian - at least one tour a day is done in either French or English, so you're likely to find an English tour most days, but don't leave it till the last day in case the tours that day are French and one of the other languages (unless you're bilingual of course!)
