At a number of points, Sheila shown that she can see things the other characters cannot
"Yes of course it is. Thats what I meant when I talked about building up a wall that's sure to be knocked flat"
Sheila uses the metaphor of a wall being knocked down to show her insightful understanding of the inspector's methods"
She knows that if they hide information from the inspector it will make things worse. The other characters do not realise this as quickly as sheila