Extremely pleasant or polite:the girl she spoke to was as nice as pie...
She would act all sweet as pie until the director shouted, ‘Action!’
"When I'm off the pitch, I will be as nice as pie. I'll sign autographs, I'll smile, but as soon as I cross the white line, I am there to win the game."
She could be as nice as pie in a few days, that's been the pattern recently.