Stars have surprisingly little leverage against studios. Only a few actors can consistently sell a movie regardless of quality--currently, only Dwayne Johnson and Tom Cruise. When stars have leverage in negotiations, it's generally because other putative participants (usually investors or producers) have hinged their participation on the participation of one or more specific actors.

I’d like to see stats on union participation among doctors and lawyers. I’d bet it’s vanishingly small, especially for lawyers.

Almost all public sector lawyers belong to a union. This includes those working for the county, state, and federal governments in a legal capacity, as well as those working at most major cities.

The same is generally true of doctors working for the public sector. Notably, in contrast to the legal field, there are a number of doctors unions for doctors employed by private sector employers.