Why, in the play, is Siobhan such a shouty character?

I am currently doing The Curious Incident of the Dog in the Nighttime as my Drama GCSE scripted piece. When I read the book and the script I interperated her as a very loving and nurturing character. I expected her to be almost like a second mother to him. Howvere, upon watching clips of the production, I realsied that the actress that played Siobhan was very shouty and aggressive. I was just wondering if there was something in her characteristics that I missed?