What you need are Letters of Intent. Contact them with your pitch, send them the script or a single page, whatever they'd like then if they would be willing to work on it (if it were funded) ask they provide you with a Letter of Intent.
This says they'll work on the film if the funding comes through. It may even include a rough price at which they'll work on the film for. Then you can show that you have cast attached without wasting their time.
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Letters of Intent are common use in bigger budgets when trying to find cast for movies in order to prove value of pitch. If the project fails to fund, no big deal. You can even call A-list actor's agents and they'll humor your pitch, mainly because it's how an agent gets paid is getting their client work. If you can get the funding, they don't really care who you are, the talent might though.
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I like to see some crew on Kickstarters, something along the lines of a Producer, Director, Director of Photography, AD, UPM, kind of above the line positions that show you can actually make this with a crew. (I NEVER CONSIDER BACKING A JACK OF ALL TRADES)
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