Does anyone think the script was originally an unsold script about an insane guy, then someone read it and said if we make it about the Joker and tie it to Batman, then maybe we can sell it to DC/Warners?
No. It was Todd Phillips' idea from start to finish. He just smuggled in a "real movie" under the guise of another comic book film. That's "the joke", and it was played on the audience -- but most still don't get that despite Phoenix referencing the idea at one of the awards shows. But Phoenix thought the film should've just been called "Arthur", and just have it be about a disturbed guy who happens to be a clown, and just ditch all the Joker stuff, but it never would've made a billion dollars worldwide if they did.
You confused me. Are you saying Todd wrote the screenplay as a Joker movie to sell it to DC/Warner so he could sell his script? It sounds like a variation of my first paragraph. Thanks for the insight
It wasn't to sell a script. It was a pitch to WB-- let's make a real movie, but I'll disguise it as a Joker/Batman movie, so people will turn out. He eventually convinced them, but they gave him a small budget to hedge their bet. They were worried that it would be too dark, and rated R.