What Chris says is true, and for a small project where the grain might be filled prior to finishing you can put on coat after coat in rapid succession. Actually, shellac, being a reactive finish, i.e., each application dissolves and mixes with the previous ones, you wind up with one thick coat.

For a large project like a speaker, and an open-pored wood like walnut, letting each coat dry and then scuff sanding to level it is a better technique. It still won't layer but each successive coat not only builds the thickness but actually fills in the pores of the wood as you go. You can see the pores highlighted with the first coat and then they diminish with each successive coat resulting in an almost smooth as filled surface.

I apply a wet coat with a pad in stripes with the grain, re-wetting the pad until I reach the end of the surface, then go back over with more stripes until the pad begins to dry out and drag. At this point, I begin padding in circles like wiping window glass; the friction of the drying pad reduces most of the ridges created by the striping and some firm pressure ensures good adhesion.

Let the surface dry overnight and then scuff sand lightly with the grain with 320 grit stearated paper backed with a hard rubber block. All the hardwares stores sell a 3M or similar block with teeth to hold the paper and it's ideal. However, tearing the 8 1/2x11 sheets in half and then quarters doesn't fit and the teeth are a pain. Simply hold the edges of the paper on the sides of the block; the rubber will keep good contact, the surface of the block is dead flat and light passes will level the surface coat yielding a whitish powder removed with a tack rag.

Now I apply another coat same as the first. 3 or 4 of these and you have a great looking finish. It's easy but not instant.

For the ultimate, after 4 or 5 days, soak 0000 steel wool in mineral spirits and use it to apply a good paste wax.