How do I choose a theorist for my solo theatre?

Choose someone you're interested in. I, for example, chose Stella Adler because I wanted to look at a woman's role in the budding years of realism theatre. This means you have to do your research. Look up lots of different theorists beforehand and see what they were doing. Don't pick someone without knowing what they do. Moreover, examiners tend to be more impressed if you can write a strong essay on someone that not everyone knows. If you choose Stanislavsky, for example, your examiner will most likely have read hundreds of essays on him before reading yours. They might be more critical because of it, especially when they know the amount of material that is out there for that theorist. So overall, make sure your theorist is unique, and you have a genuine interest in them as a person, as well as their theory.