As with most things in film and television production, it largely has to do with money and status.
A producer is someone who has responsibility for managing and coordinating the talent (actors, senior writers, directors, cinematographers, editor, et cetera), labor (set builders, electricians, effects, et cetera), and the support services (the “office” staff such as accountants, production and personal assistants, catering, entertainment, et cetera) as well as typically overseeing the marketing of the film, all done to a stated vision of the intended result within budget and schedule (hopefully). Producers generally work for or are hired by a production company, and in some cases may be a partner or owner in that company. Although a producer is responsible for the overall operation of production, they may or may not actually oversee day to day operations or be on set for filming depending on the size of the production and whims of the producer.
An executive producer is someone who has a material investment (either financially or creatively) in the production but no specific duties. The traditional role of an executive producer was to bring in financing (either their own money or to scout investors in the production), but it has become common for someone who as a substantial creative interest in the production to be titled as an executive producer, e.g. the star of a long running television show or film series. An executive producer may also have used influence to get a production or script made on behalf of someone else, or arranged for distribution domestically or abroad, even though they had no other material or financial interest in the production. If you look at Martin Scorsese’s IMDb.com page, for instance, you’ll see that under Producer credits the majority of his credits are actually as an executive producer in films that he otherwise had no involvement in.
A showrunner (sometimes credited as “supervising producer” or “co-producer”) is a rather vague description given to a senior production assistant on a television show who is generally granted authority for managing the actual day to day elements of the production, especially during writing and filming. Showrunners often come up through the writers’ room or at least have a background in writing, and have facilitated the kind of seasonal-arc storytelling that has become popular in the last couple of decades by enforcing a certain style of dialogue, direction, set dressing, and fashion that was popularized by The Sopranos, The Wire, Mad Men, et cetera.
An associate producer, to quote David Mamet, is “what you give your secretary instead of a raise.” This isn’t actually true as having an associate producer credit may entitle someone to a higher salary or more residuals (in the case of an actor or non-staff writer), but it does not convey any particular authority over or ownership in the production.
These titles are obviously kind of fungible and don’t necessarily reflect all or any of the responsibilities of the holder. There are many cases in which a producer may take over direct creative control of a production (for better or worse) and either fire or sideline a director, and many cases where people carry multiple titles and the responsibility between them, or split the work in a teamwise fashion. For instance, the Coen Brothers handle most of the creative elements of the production (production, director, writers, sometimes cinematography, and editing), and either divide or share the titles.
So, basically, the executive producer is someone who has helped get the production financed or developed, and may have other roles but is not in charge of the management or creative content by virtue of having that title.
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