Becoming a Background Performer
Background performers (extras) do not have any lines, but taking on this role is a good way to see what happens on a real TV or film set. You will likely be given some time of action do when filming. This may be something like faking a conversation (known as pantomime). Your lips move, but there is no sound.
There are casting agencies that will help you find work, but do your homework as most charge a registration fee. Professional head shots are not required, either. Some agencies may ask for a 3" x 5" or 4" x 6" photo.