A few years ago I had a flurry of people who wrote to me about this topic in the space of three weeks. There must have been a story about Talent Agencies on the Today Show or something.
Basically I have no firsthand experience with managing talent. Working on films has placed me in situations where I had to coordinate with Extras Casting and the agents of actors, but a talent agency is another matter. Talent agencies normally manage performers of various skills and get them bookings.
If, however, we are discussing filmmaking then I have some thoughts. For instance, a casting agent must already have an established reputation. Nobody is going to take chances with a newcomer. The handful of casting agents who I’ve worked with here in Atlanta built their own reputations over the course of many years. A casting agent must have the gift of spotting true talent that will shop up onscreen. I recall times that the director of Extras Casting would go “shopping for people” just to find someone with the right “look” to fill a 1920’s church scene. A knowledge of how films are made is necessary for a casting agent to be of any use to a production.