There is a small risk of accidentally providing an electric shock to a bystander if they are touching the body of the casualty as the AED releases its energy. Generally, however, the electricity will travel between the two pads and not elsewhere.
Before pressing the button on the AED to deliver the shock, it is important to give a clear “Stand clear!” command and look all around to make sure nobody is touching the casualty. When you are happy that the area is clear, give a further command of “Shocking” and press the button.
Concerns over rain, puddles or metal surfaces are generally unfounded, although it is good practice to dry the chest or move the player out of large puddles if able.