This question already has an answer here:
- Show OPs if someone is answering, live 2 answers
When someone is typing an answer, the OP (and only the OP) could view something like this:
This would tell me if someone was answering my question, and would be an indicator of whether I should hang around with the page open, or just close it and check later.
Related, not duplicates:
Because they suggest everybody could see a list - rather than just the OP and some animated bubbles.