The professions where Mercury in Leo comes into its own always contain a stage. Not necessarily one made of boards, but always a live audience — someone listening right now. Teaching, public speaking, presenting programmes, authored courses, being the face of a brand, selling through personality, politics, directing, acting, coaching in sport. Anywhere you can speak in the first person and read the response in someone's eyes. In my experience this Mercury is at its strongest the moment there's a real listener and a real reaction to play off.
What this mind does especially well is wrap meaning in a memorable form. Advertising, scripts, lectures that make a hard subject land with a wide audience, authored blogs and podcasts, talks at conferences. It can turn the complicated into something simple and sticky, and that's a rare skill people pay well for. Quite often these are the people who become the face of a project before they've become the expert on its substance, and then spend a good while catching up with themselves on the detail — which, handled honestly, is no bad arc to be on.
What goes less well is the long, monotonous, solitary slog: working through spreadsheets, template copywriting, bookkeeping, any task with no feedback and no spectator. Without a response this Mercury wilts, starts to put things off, switches to almost anything that gets it back in front of people. I'd put it this way — for someone with this placement, choosing a career is first of all a choice of format, not subject. They can grow to love almost any topic, as long as there's someone at the end of it to tell. The pull towards status, recognition and a visible authorial signature isn't vanity here; it tends to be the condition under which the whole personality assembles. Read it lightly, as a way to understand your own wiring, and treat the rest as your own to decide.