This is John Williams’ second appearance in Inspirational Art. Given my love of Pre-Raphealite painting and background in the SCA this shouldn’t be a surprise.
The Magic Cirlce is a particularly interesting painting for RPG inspiration. Well into the 80s our image of what the middle ages, and thus much of fantasy, drew heavily on Pre-Raphaelite imagery. This particular painting, given the simplicity of the subject, emphasizes several of them. He gown, a simple form with a pale but not pastel color, mid-forearmed sleeves, wide facing on the collar and cuffs, is a classic fantasy image. The multiple winds of the belt with the dangling front is classic although Waterhouse paints more of a sash here (perhaps an influence from India parsed through the Victorian eye). The cauldron directly on the fire is another image well known.
While all are good for a GM’s use in describing an NPC I’d like to draw particular attention to her implement. The rod, which she appears to be using to inscribe a circle, is neither the staff or dagger, the traditional magic implements in fantasy literature. While D&D has many rods I rarely see them used and almost never see the rod as a standard magician implement in adventures or books. The inclusion of the cauldron inside the circle is also unique compared to more modern fantasy art, writing, and games.
Do these differences represent her specialization or a quirk of personality? Does she wear a sash belt instead of a leather for a reason? What is she brewing that requires it to be protected from the outer world during its enchantment? Why does she use a rod and is it metal, wood, or some other material? What is the decoration on her skirt?
Finally, this image is used at Wikiquotes to illustrate a quote from Good Omens:
Precisely because she was a witch, and therefore sensible, she put little faith in protective amulets and spells; she saved it all for a foot-long bread knife which she kept in her belt.
I see inspiration for an encounter, an interesting NPC, or even an order of magic. Regardless of which you choose, Waterhouse has provided some excellent inspiration.