So, me and some buddies from college are going to be making a music video to Imogen Heap's song "Hide and Seek." One of our locations is going to be a HUGE theatre. We don't have enough people to fill the whole theatre obviously, but I was thinking that this could work. Would it work if I just locked the tripod down and shoot a background plate of the whole theatre being empty then have the people (25-30 people) sit in the first few rows, then switch them up, have them change into different outfits, and then move them back a few rows. Then do that over and over again until they reached the other side and the balcony. Then when we go into the edit, just composite them into the original background plate. Would that work to make it look like that the theatre is sold out? If so, would it also work to have a moving camera such as a dolly shot going behind the performer and showing the crowd in the "pit" doing the same procedure as I did with the camera being locked down? THANKS!

I explored this effect over a year ago for a short film. The overlap of people's heads on top of other people in the row behind them is extremely tedious to matte out. Imagine tracing the outline of a person for, say, one second, or 24 frames. Now imagine doing that for an entire theater of people.

However...
Shooting the sections in order of perspective from the camera makes this effect possible. Make sure the only overlap between people is being done live, and by shooting dead on you can successfully trace out the columns or diagonals along the sides of the seats with little to no overlap. Not sure if you follow that...Basically trace straight lines with the camera lens as the origin, and each shot should fill the seats along each line. This way you aren't forced to composite one person over another, only seats, which don't move as much

Similarly, if you are looking for a shot of the audience from the side in the aisle, then use rows. Shooting a tighter shot from the front or back, use columns.

Also it is imperative that you lock down shutter, gain, aperture, focus and white balance. When matting out, remember to feather the matte edge.
Good night and good luck.