During the first "Janet -Dr. Scott-Janet-Brad-Rocky!", Janet responds to Dr. Scott, but the first person shown going "Janet!" is Brad.
At the start of the film we see the projectionist start a single 35mm projector loaded with a single 1000 ft reel. This would only provide about 20 minutes of the film at most and nowhere near enough footage for the whole movie.
Brad says their car broke down a couple miles up the road and they walked to the castle. However, when they get there they aren't worn out or sore from walking those few miles. They should at the very least be sore from walking that far.
At the end of the song "Time Warp," the drummer can been seen trying to find a place to fall down like the rest of the cast does.