lawyers, priests, and physicians are professionals. they answer, in theory, to a higher code of ethics than whoever they're getting paid by at the moment, partly because they are in a better situation than their client to judge whether they've done their work well or poorly. their respective professional colleges judge that, largely to protect the clients from incompetent or corrupt professionals. partly as a consequence, it is often considered questionably ethical for them to become too dependent on a single client. they have duties to their profession such as education and the advancement of the profession itself—raising the level at which their successors can work
(those are not the only professions, but they are central examples of the professions)
this strikingly contrasts with the relationship in capitalism between employers and employees; like professionals, employees assume an ethical duty to act in the interests of those who pay them; but, unlike professionals, employees must defer to their employer's judgment on what that means. as a consequence, it is often considered questionably ethical for them to not be entirely dependent on a single employer
freelance artists, such as freelance actors and freelance photographers, do not fit either of these categories very well. it's not clear that they have a duty to act in the interests of their clients at all, but their clients often may not be able to tell how well they've done their work. moreover, both professionals and employees usually have a duty to suppress their own feelings in service of their work, whereas artists' work is their feelings to a significant extent. certainly there is no professional college of actors needed to protect clients from incompetent or unethical actors; such a thing would be absurd. the sag, as its name indicates, exists to protect actors from their clients, not vice versa. it is a cartel (look the word up before arguing with me about this), not a professional college
so to me it seems like a category error to describe welles's behavior as 'unprofessional', like trying to assess the electrical conductivity of a social movement, or the color of television