I really like Guy's use of light and how he models his forms. He used a limited palette for this painting, there's a presence in the painting that I think is remarkable. He captured the moment so well, you can sense her excitement or surprise as she enjoys the show. Once again the object of a master study is not to replicate or complete the image but to learn from the artist about what they understood about the principles of art and design. He uses the subject's hands to perfectly balance the visual tension between the portrait's head and the hands - really textbook precision if you study the theory behind portraits (Composing pictures by Donald Graham is an excellent resource for these principles and is one of my favourite books on my shelf, though in my opinion the level of the book is aimed at intermediate to advanced artists since he deals with some very meaty and advanced viewpoints on the subject). I think the use of dramatic light is the thing I got most out of doing this study. I do studies to keep my eye in and keep my illustrative skills sharp.
You can see a digital copy of the original I used as reference here.