User blog comment:Sachoras/Cersei - friend or foe?/@comment-5670832-20121221023011

Cersei is complicated. I think she did get drunk during Blackwater and stopped caring about seeming regal and an example to others. Her self control and emotions seem to be tied to how well or badly things around her are progressing according to her plans. As for Sansa, I really think that Cersei wanted Joffrey to marry her, because she thought Sansa was stupid and insipid and in no way viewed her a threat to Cersei's power. She especially wanted them to marry when Sansa was in love with Joffrey and was making excuses for Joffrey's failings, but Eddard's execution made Sansa hate Joffrey. Even though Sansa always said the right words, she was not a good enough actress to make anyone believe them. Maybe by "Blackwater", Cersei already suspected that Joffrey wouldn't be marrying Sansa, so there was no more point in being nice.