Thanks! The Hound is my favourite movie in the series.
I believe, in one of her interviews Kriuchkova said she was pregnant at this time (btw, Yuri Veksler was the father)) and she simply couldn't afford to cry so much as the role would demand so she had preferred to laugh. I don't really see is as a fault, however: Selden is an obviously insane person, and it's only natural that his sister is a bit cranky, too.