Observe for true talent. Many actors make it based on luck, physical appearance or charisma. However, you want actors who are going to be in it for the long haul. An actor with a twenty year career can earn you enough money to retire well before he does. The first and foremost factor when scouting for talent should be actual talent. Remember that regardless of what your peers may be telling you.
Look for versatility. An actor who is willing and able to play both comedy and drama is appealing. An actor with multiple talents means multiple avenues for money making. The ability to sing and dance is also desirable.
Listen to your gut instinct when it comes to talent. Talent is in the eye of the beholder. You likely have a strong eye for it if you are pursuing this as a career. Go with your first impression unless the actor does something to make himself worthy of a second chance like being unbelievably charming or giving a perfect performance.
Consider the look of the actor. If he is a great actor with a gorgeous face--and also happens to be severely underweight--it can be tricky to cast him. Lead actors get funny about having very attractive co-stars in lesser roles. Someone with a beautiful face and imperfect build can be hard to cast. It certainly does not mean that you shouldn't give him a chance, but you should be realistic with yourself about how many auditions you'll be able to send him on. If you think he is better of starting with an agent who is lower on the rung than yourself, give him a referral to someone if you can. If you find true talent, try to never leave him in the lurch.
Typecast the actor in your mind. All actors hope to never be typecast, but the business is comfortable with typecasting actors. Good girls are often given that role repeatedly. Rebels tend to get cast in all kinds of crazy roles. It makes the business people comfortable because the audience gets comfortable with that sort of image. Imagine what sort of typecast the actor would fall into if successful. Ask yourself if that's a marketable position for the actor.
Talk to the actor. You can get a true read on an actor when he talks about himself and his career. An actor that is completely egotistical is sure to grate on the nerves of any director at an audition. He'll never go anywhere. An actor who lacks any self-confidence is also a dangerous talent to gamble upon. You want affable, professional actors who are going to represent you well in the casting office. A client with a bad attitude can poison your reputation at a company forever.
Attend as many plays and showcases as possible. Note down great actors in bit parts when you go to the movies. Try to contact them when you get home. Most actors who appear in feature films--even in small roles--have representation. Not all do, though. Your response will likely be met with enthusiasm. Bit players especially like to know that their work was seen, acknowledged and appreciated.