From Studio Guide 2010: Casting Recommendation.

I received Studio Guide 2010 from my PDN subscription this month. And this is the first time ever that I have the issue Studio Guide in my hands. I cannot begin to tell how many studios and the advantages and equipment that are available around the US and internationally.
However, one of my favorite things that I read here was an interview made to Edward Kim from HOUSE CASTING:

PDN: Do you have any tips for photographers who are attempting to do their own casting?
EK: Hire a casting director. If it's a budgetary issue, a casting director will negotiate better rates for you and know how to find available talent or make talent available. Otherwise it's just about knowing what you want and having realistic expectations and communicating very clearly with the agencies.

So, I recommend that whenever you are presenting an estimate for a client for production costs, to include in your estimates, a Casting Director. He/She will care for you and will of course avoid you the horrible and painful calls over and over again.