One of the things that I find interesting about Wyrd Sisters is that it shows Pratchett feeling his way into a significant part of Discworld. While it's true that he's already made clear from the first that this is a world with Magic, and that there are witches and wizards, it seems to me that he has decided by this point that wizards will play a relatively minor part. In fact the wizards of Equal Rites bear only a general resemblance to those in Sourcery and I've always thought that Sourcery had the feel of something that happened in the distant past--except that Rincewind plays such an important role.
For whatever reason, Pratchett has decided to develop the witches as an important factor in Discworld, though they seem to be found primarily in the Ramtops. In this novel, we get an introduction to Granny (in some of her less attractive ways), Nanny (who changes very little) and Magrat.
Granny doesn't want anyone questioning her, or challenging her powers and understanding. But in this novel, she is almost embarrassingly ignorant. She has no idea what the theatre is, or what actors are. But she is not about to admit it--especially to Magrat. Nor does she understand drama at all. I remember the first time I read it, I was a bit embarrassed for her in her behavior at the play. Nanny doesn't know anything about the players and drama either, but she keeps her mouth shut.
It seems to me that Granny is particularly hard on Magrat (with some justification) because she is the "New Witch on the Block", but she is far more talented and powerful than Granny is willing to concede. Magrat has considerably more guts and perception, at some points in the book, than Granny does. She has tried to get the coven formed, though she's in awe of Granny. And having met the Fool in the meadow and fallen for him, she asks what his name is. The two senior witches basically discount him and don't really see him. Magrat calls them "a pair of silly old women," and says she's going home. And this leads to the quarrel between the two. And it is Magrat who actually save Nanny by the spell which Granny tries to say probably won't work--but it does. Granny is so sure of herself that she deludes herself about TomJohn and his destiny to be King.
Magrat is, in a strange way, a threat--which explains some of the reasons Granny treats her like a child. But it is Magrat who spots the resemblance and thereby saves the Kingdom of Lancre.
And it is Verence the Fool who is accidentally, the means of almost defeating the witches power, because he introduces the magic of words. Words are important to Pratchett. Ideas about good and evil and what makes things good or bad are important to him. And he will use the witches to explore some of this.
But, it's clear that Pratchett was working quickly, and making small errors (not caught by his publishers) at this stage. For example, Verence (as ghost) see the coach being pulled by his own horse go tearing out of the castle when he goes looking for the baby he believes is his son.
But one horse has turned into multiple horses by the time it arrives at where the witches are. And while Granny works the death of the primary confederate guard and sends him off to be a sailor, they all go home and apparently leave the coach and horses where they were meeting.
It's a fun book, but it's not as serious in it's theme and exploration as his later witch books will be.