There are two methods of giving a person artifical firebreathing. One way is a more natural way: outfit the body with a way of getting rid of gasses such as methane and other flammables, then have a small spark dispersal system. When the gas comes up, the spark hits and flame erupts out. The inside of the mouth is, of course, made incredibly fireproof. The other way involves turning at least part of the back of the head metal. What they do here is have a plasma flame dispersal system. Usually, the dispersal system is so perfect that it will balloon out as soon as it has reached the end of the mouth but not a moment sooner.