I had the idea for this painting after a couple of things I'd read about Mary towards the end of her incarceration.  One was that when Elizabeth was deliberating about whether or not to order Mary's execution, she apparently was heard, while pacing, muttering "Suffer or strike, strike or be struck".  She had major qualms about being responsible for the execution of an anointed queen while at the same time believing that she was in mortal danger from a plot in which the trial had posited Mary had been instrumental.   Also, while Mary awaited news of the date of her execution, legend has it that the night the decision was made, Mary's chamber was illuminated by a blinding light, light enough to read by, three times.  Since then astronomers have suggested that a comet would have been visible in the sky at that place and time.  So a less mystical explanation but in medieval times comets were believed to be portents of a significant death.  
I wanted the dress of the figure to be redolent of muscle, sinew and bone and organs.  The figure in the foreground just put himself there.  He just came out like that.  I know he looks a bit like Knox, and I had tried to disguise him by giving him a crow's head but it didn't work so I went with what instinctively felt right for the painting.   Similarly the horse in the background just seemed to have to be there.  Sometimes bits of a painting just paint themselves, and I don't always know why.