Something very important to keep in mind when you approach a
working actor - especially a recognizable one. This is what they do
for a living. Many are willing to help out a low budget moviemaker,
but this is much more than a hobby to them - working for the SAG
ULBA or on deferred pay takes them off the paying market for the
length of your shoot.
As knightly mentioned, a well known actor gets a lot of scripts.
First on the list are offers at or above their "quote". A guaranteed
paycheck.
Second on the list are scripts from studios and producers for less
than their quote - but these are also firm offers (money is attached,
they get paid when they say "yes"). These are projects like CRASH
- where the script may be more important than the money - but
there is still money (and often a high profile writer, producer or other
actor) attached.
Third in line are pet projects. The star has a pet project they will take
to the studio or producers with no money attached, but it's a story
they really love.
Fourth in line are scripts from friends or friends of friends. No money
attached, but at least brought to them by someone known to them
personally.
Eventually we go all the way down to some total stranger with a script
and no money. Imagine the odds of doing that script instead of any
of the others?
Knowing that you are number five on a very long list of scripts being
offered should help you decide how to approach agents. Maybe you could
offer a producer credit to the actor, or a good friend of the actor. You
might have to do things the old fashioned way. Get the script to a producer
who can make a firm offer to a star.