This is going to be the same process as if you were hiring someone to be on your payroll within your company. The only difference here is they are a freelancer, not fulltime and the work is generally temporary. With freelancers you will want to be sure that they have all their own equipment, reputable references, liability insurance for their business, cameras and self when on the job, and know what you’re looking for in a finished product. These are all appropriate things to ask during an interview, and should be discussed and verified before hiring or paying a freelance photographer.