So, Hollywood really likes to make scientists and mathematicians write on Plexiglas (or mirrors, or windows, or other transparent or reflective surfaces). Presumably it's so they can get shots like this one:
With a face behind a wall of equations - no mathematician or scientist I have ever known has done anything even remotely like this.
I'm curious to know how this got started. The oldest example I've been able to think of is the scene the image is from, in Sneakers (1992), but that's still pretty recent...
This TVTropes page is tangentially relevant (and has a few examples buried throughout).