The reason Patti was drinking champagne
SPOILERS
The reason she was probably drinking it was because she was celebrating (thinking she was the only one alive other than Sam and the part was now Patti's!) She didn't think Sam had a chance to get the part from what Stryker said, so that's why Patti wasn't going to kill her. But once Sam admitted killing Stryker, Patti killed Sam because no one was going to get the part now that Stryker was dead.
I hope Patti got to enjoy some champagne before they took her to the mental institution!