The Producer (if any) or photographer will usually help to source talent, either freelancers or full-time from model agencies. Unless the client wants to do the liaising themselves the production company (photo or video) will handle it. Then there will be a lot of to and fro with client on which talents to select etc. It's faster if the client actually does the work themselves by cutting out the middleman, the photo company may or may not charge the client for the service, and there is normally a markup from talent fee.