In Hollywood, it's the job of a director to make movies and it's the job of a producer to make money. These jobs aren't necessarily at odds, any more than contractors and homeowners have to be at odds when a kitchen is being remodeled.
Anyone who has gone through that experience knows how things turn out way too much of the time.
Since the very early days of movies, there have been two things producers have coveted, bankable stars and successful franchises. For a bankable star, the producer imagines members of the general public saying "Let's go see the new George Clooney movie." instead of saying "Let's go see the new movie about a lawyer." For a franchise, it's assumed the public says "Let's go see the new Friday the 13th movie." instead of "Let's go see a slasher film."
Can we tell if it's a bankable star or a franchise? No, not always. In the 1920's, Charlie Chaplin was by many accounts the most famous person in the world. A wonderfully adept comic actor, he could draw throngs of people to any new film he put out.
As long as he wore the hat and the mustache and carried the cane of the character known as The Little Tramp.
Oh, yeah, and he had to walk a certain way.
Was this a franchise or was he a bankable star? Because the audience knew the actor's name and would accept no substitutes, the general consensus is Chaplin was a bankable star, one of the biggest in the world. There were a lot of actors who audiences only wanted to see doing one thing, or possibly the actor might get lucky and stretch the boundaries, but in his heyday, Chaplin and The Little Tramp were inseparable.
Chaplin also pulled off one of the great tricks in Hollywood, going from bankable star to bankable director. After the silent days were over and most silent stars were long gone from the business or reduced to being character actors, Chaplin could still get studios to give him money to make movies where he was either the director or the star or both. He wasn't the colossal bankable star he was in the twenties, but his career arc is much more positive than folks like Buster Keaton, Gloria Swanson or Mary Pickford.
A bankable director is a better long term investment than a bankable actor. This is certainly an easy story to sell, because there are many examples of bankable directors having long careers with tremendous successes decades apart. But people also forget the bankable directors who had their brief time of glory only to fade away quickly.
At the end of the silent era, many of the directorial stars faded as quickly as the people in front of the camera. D.W. Griffith was the top of the heap in the silent era, but the talkies did him in. Likewise Mack Sennett. One star director whose name meant spectacle in the 1920's and meant the same in the 1950's was Cecil B. DeMille. There were also some up and coming directors who started in the silents but became stars in the talkie era, most notably Alfred Hitchcock.
Through every era, there are star directors. John Ford, John Huston, Howard Hawks, and David Lean all had long careers at the top, but that era also produced some shooting stars like Preston Sturges, Busby Berkeley and Douglas Sirk. Today, we have Spielberg and Scorsese and Lucas, but what about Francis Coppola? His star isn't nearly as bright as the others who started with him, and Lucas' star is stuck to the Star Wars franchise as tightly as Chaplin the actor's was to The Little Tramp.
Some star directors don't really promise huge hits. Woody Allen has had long relationships with a few producers because he makes films that are cheap by today's standards and still have a market. The Coen Brothers started similarly, but have made some forays into the big budget films with mixed results. Again, Nobody Knows Anything.
You can't break up a bankable team. This is conventional wisdom that has been stood on its ear several times. The perfect example of this being true is Abbott and Costello. A major counterexample is Fred Astaire and Ginger Rogers.
Astaire and Rogers made a string of hits in the 1930's, but Ginger decided she want to act instead of dance and Fred went and found other partners. Fred was a bankable star as long as he promised to dance, which extended well into the 1950s, and Ginger became an Oscar winning star of what are now called chick flicks. For them, it was a win-win.
But even in win-win situations, often someone wins bigger. Ginger Rogers' career began to fade by the end of WW II, and she showed up in smaller budget films, sometimes as a co-lead with another actress, then a third bill, later playing the mother of the main character. This is often the trajectory of bankable female stars in Hollywood.
Bankable female careers don't last as long as males. This is still conventional wisdom. Women become bankable when they are young and pretty, and as they grow older, audiences want to see younger, prettier women, and Hollywood always has a surplus of them.
A two word rebuttal to this well thought out narrative: Joan Crawford.
And another: Bette Davis.
And, oh yeah, Katherine Hepburn.
Meryl Streep, for pity's sake!
Another thing that changes this rule is people taking better care of themselves so that the audience does not have any problem believing that an attractive lead actor actually wants to have sex with 40 year old women such as Cate Blanchett, Julia Roberts or Salma Hayek.
Another example from an earlier era is forthcoming below. It's a good rule of thumb, but it's also an example of Nobody Knows Anything.