The Cassandra sketch is famously the only frontal image of Jane Austen we have. A number of commentators have said this Jane is seriously annoyed. She resents being made to sit still while her sister draws her likeness. Defensive, cross, she folds her arms and stares sulkily into the distance.
Apple sauce. It is too far out of character, for both women. Would Cassandra force her sister to sit still, like a naughty child, and then deliberately sketch "Angry Jane"? Would that be the kind of sketch the Austens would love to hang on the wall and look at every day? If Jane were not in the mood, would she not have explained that to her sister, and ask to put it off till another time? Or, if Cassandra were really so assertive, wouldn't she insist that Jane adopt a more elegant pose and look at her properly? None of it makes any sense.
Let us take another, more careful look at the sketch, and see what we can deduce from the detail.
The first clue to a proper interpretation, ironically, is the chair. You can just see the back of it around Jane's right side. It does not look like a sitting-room or occasional chair; it is more likely a kitchen or dining-table chair. The setting, then, is probably a mealtime (as far as we know, Jane never used the dining table for writing; it was not sufficiently private; also, she had a writing-desk). The meal has just ended, or perhaps is between courses. Jane is relaxed, sitting back in her chair. She has folded her arms, as one does at such moments. From across the table Cassandra is sketching her; evidently, Jane does not object. The conversation is flowing, and she is giving her attention to someone on her right. She is amused. Look at that face (second clue) - are you sure that is a frown? Is it not rather an incipient smile, such as every face wears at a convivial dinner party? This was surely typical Janey, gorgeous Janey, and Cassandra could not resist to capture it. It is unfortunate that nobody approved of the likeness (evidently, a "driver's licence" image!). Nevertheless, there must be a reasonable resemblance, or Cassandra would not have gone as far with it as she did.
Jane Austen was said to be very 'conversible'; her chat was witty, lively, and interesting. But she took equal pleasure to listen and observe - as she is doing in this sketch.