I have no first hand knowledge at all, just what I've read in screenwriting books. I've read -- Don't go through the agent, if you have any other way -- they only get their cut if the movie gets made and / or the star gets paid.
I spoke with a casting director at length about this issue.
What she told me is that you have to have funding in place before you contact a name actor. They won't tie up their time and commit to a project where the funding is not already in place. She also said that if funding is in place, it would be easy to obtain known actors.
Now, if you are wanting an unknown, that's a different story.
When you say star I assume it's not an unknown you seek. I was told by a prolific indy producer last week in LA that she goes to the stars agent but not the American agent. Apparently most "stars" have agents in London. They are apparently much more open to this kind of thing. If that doesn't work for you, I suggest trying his/her manager.
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