In a previous discussion it was pointed out that "bereits" refers to a condition whereas "schon" refers only to time. Because I am unoriginal I'll use the example the previous discussion used:
If you are upstairs in your house hiding from the zombie apocalypse you would say, "Sie sind bereits im Hause" However, in a much more peaceful time, if you are waiting for your friends to come to your place for dinner you would say, "Sie sind schon im Hause."
Zombies already in your house is a condition, whereas friends arriving for dinner is about time.
At least that is what was discussed. Sounded legit...