Did I not say? Since you cast them against the shape of your teeth they will have a reverse mold of your teeth inside them. This fits right onto your teeth like a glove. If molded right they should stay on no problem.
If you have a good mold of your teeth to work on you should have no problems. I wore mine a whole lot during Halloween. I could talk and drink water with little discomfort. No colored liquids or food though.
If you do make them and find they are loose, just mix up a little more of the acrylic into a little ball, drop it in the tooth cavity, and mash it back onto the teeth mold (with the separator on the mold first). If that doesn't work, you could do the same thing but fit it to your mouth directly. This is similar to how you would fit commercial fangs.