• champagne bottle
• champagne corks that have been carved to fit into the bottle w/o compressing
  the flange (you should be able insert it by hand, but the cork should fit tightly enough to
  hold until the actor pops it.--I could never buy the mushroom shaped ones, but usually could keep a supply around by asking the partyers in the company to save them for me.)
  
• chilled club soda or seltzer water
  
• coca cola
  
• champagne foils
  
• champagne wires (these last two make opening it take longer. usually the director will cut the wire to speed things up. if you use the wire, get a rebar wire twister to install the wire. This is a hook on a swivel with a handle--hardware store. It allows
  you or the stage crew to twist the wire on tightly, quickly and neatly.)

it is important to prepare the champagne as close to the time it is opened onstage as possible. Preshow or intermission, depending, usually works okay. One regular bottle of champagne takes a teaspoon or so of coke, then fill with bubble water pouring the water carefully down the inside side of the bottle to preserve as much bubbly as possible. It may be better to put the coke in last, if it wants to foam from the sugar. Cork, foil and keep cold.

This worked reasonably well here in many shows. Nice pop, fizz in the glasses, and not too much spray (which is not how champagne is supposed to open anyway). If the director wants it to spew out, try shaking it a bit before it goes onstage and make sure the cork is tight.

The recipe probably really only needs a few drops of coke--its really for the color. Use diet if regular makes it too sticky.

None of the paint-on products seem to work well. They all slide down and become denser toward the bottom, in a way that real liquids just don't. Filling the glass with a coloured clear resin gives a believable effect, but it's messy and tricky. Use EPOXY resin, rather than polyester casting resin, as polyester resins shrink ever-so-slightly when they harden and pull away from the sides of the glass just enough to spoil the illusion. It's hard to describe, but instantly obvious when it happens. Epoxy resins don't shrink. All resins build up static charges, however, and if you're trying to pour it neatly into a glass, especially a plastic one, this can cause the stream to veer over and glop onto the side.

My current favourite method is to cut a piece of lighting gel to just fit inside the glass (not overlapping), and hold it in place with a bit of clear tape. The palest amber or gold gels are good for champagne. Don't worry if the sides of the glass are slightly curved and the gel isn't. From even a few feet away this isn't noticeable, and it's still far more convincing than anything else. Until someone raises the glass to drink from it, that is...