The question of how to communicate professionally with clients is something that should be taught alongside drawing skills and almost never is. Here is what I've learned.

Email communication: be brief, be clear about what you need, confirm understanding before doing work. The most expensive misunderstandings in commission work happen when I assumed I understood the brief instead of confirming.

Revision communication: when receiving revision requests, paraphrase them back before acting on them. 'I understand you'd like the background darker and the character's expression to look more determined. Is that correct?' This prevents wasted work and demonstrates professionalism.

Deadline communication: if a deadline is at risk, communicate early. A client told three days in advance that delivery will be two days late is manageable. A client told on the delivery day is furious.

Conflict communication: most client conflicts escalate because of communication failures, not artistic ones. When something goes wrong, acknowledge it, explain what happened, and propose a solution in the same message. Don't make the client ask for the solution.