Hi Laura,
Camera Shifts are a very good friend of Stop Morion Animators all over the world.
When I was working in London on Frankenweenie, I had the worst camera shifts ever. The camera shifts for many many reasons. How much the temperature changes over night is the biggest culprit. The sets actually breathe during the day. When hot lights are turned on the set will begin to heat up. The warm studio at day will turn dark and cold at night that cause the tables to warp. The sets expand during the day and contract at night. I had to wait 3 hours while the camera crew put a car jack under the set to try and make it match the last frame taken. There are ways to correct this in Post production. There are many reasons for shift.
Tripod on carpet.
Wooden floors.
Cats. (Huge problem)
Dogs (not so bad)
Tape slipping could be it.
People walking around.
The list is endless. I've been in studios that were made out of solid cement and the camera still shifted.
The one solution that works is to start the shot and animate until you are done. That is very very hard to do on a long shot
It just happens. Don't worry too much about it. On animating to dialogue vs "dialoguing" to animation: I'd vaguely remembered seeing special features for some animated films where actors were clearly recording to an already in-progress (and sometimes finished) animation, so I was asking what the case was for stop-motion specifically. (the answer is pretty obvious in hindsight, but I was making sure) About your question on dialogue.
The voice acting is always recorded first with professional feature and commercial work. The voice acting is what inspires the animator to create the perfect performance. Mr Fox would have had a different performance if George Clooney had not been the voice. The only time the voice is done post animation is when the Big Suits who are funding the project decide that the voice needs to be changed. That is what happened on "Jame and the Giant Peach" Disney changed the voice of the Centipede during animation. The new voice was the actor Richard Dreyfus.
He had to re-record the lines to the already animated sequence.
The voice comes first then the animation is created from that performance. About securing stationary parts of a puppet (that aren't the feet) to prevent them from moving while the rest of the puppet is animated.
A good tip for keeping things in place is to use wire.
If your box is hollow from the back, you can drill two small holes then loop the wire around the wrist or the hip. Then just twist the wire to secure the body part to the box. Say you want to tie down the wrist. Drill a small hole on either side of the wrist then feed each end of the wire into the holes. You now have a loop around the wrist and each end of the wire through the holes. Just twist the wire to secure the wrist to the top of the box. Again, you have to be able to reach inside your box to do this kind of wire work. You also want to make sure that the Box is extra secured to the table top.
You can also Pre-Drill holes so that you can tie-down the bits as you go along.
Cheers,
Chuck Note: for both my "armature on box" animation and my "character dialogue" I was using props that I was unwilling/unable to to drill holes into the thread the wire through. In these cases, a little-bit of strategically placed duct-tape helps, but isn't 100% secure. I also spent a lot of time tracking and preventing the frame-to-frame wander of these stationary parts. That's not perfect either, but in most cases there is a certain tolerance for believable shift, since the weight/momentum of other parts would cause some shift to your stationary parts irl.