. Before 1990 or so, didn't they actually wait until the stinking film was finished?
Nope, they didn't. In fact, back in the day, it was not uncommon to come up with a catchy title and movie poster
first, and
then make the movie if you got a good response to the poster. (See, for example, pretty much every horror movie produced by RKO back in the 1940s, including such classics as CAT PEOPLE and I WALKED WITH A ZOMBIE.)
And the practical reality is that you usually have to start marketing your movie
before the film is finished, so the people producing the trailers (who are NOT the same people making the movie) have to make do with whatever footage is available at the time. But there's no way to tell what shots are going to make it into the final cut of the movie until much, much later. (I was just reading today about an entire sequence that was shot for BRIDESMAIDS that ended up being cut, despite being very funny, because the movie was running long.)
You can't wait until the movie is finished. If anything, the audience is even more impatient to see trailers these days than they used to be. Can you imagine all the grumbling and fretting there would be on the internet if the studios didn't release any trailers until the movie was absolutely, positively finished, which would be right before the movie opened.
"SPACE VIXENS opens in only six months--and we still haven't seen any trailers yet!"
"Well, they're still doing editing and post-production, you know. And there's that new rule that says they can't release a trailer until the movie's final cut is locked down . . . . "
It's not like most movies are finished a year or so before they open, and are just sitting around on a shelf somewhere while the sales and marketing campaign gets underway.
(For what it's worth, this happens in publishing too. I've written jacket copy and commissioned cover art for books that were still being written. Usually it doesn't make a difference, or you can fix any discrepancies before you go to to press, but sometimes the cover doesn't exactly match the book--because the cover was finished before the text was. Unless you can persuade the author to change their book to match the cover!

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