That way, if nobody else showed up, Billy and Jeff could still make the film themselves. So they were pretty flexible. They could shoot whenever, wherever, and didn't have to rely on too many other people. They considered casting other actors to play David and Spy, but, in the end, it just didn't make sense. They'd have to work around actors' schedules, and if someone dropped out along the way, they'd be royally screwed.
Most people underestimate just how long it takes to make a feature film.
Especially when it's being made in a garage. Besides, they were both working, professional actors in their own right. And Billy and Jeff knew that they were in it for the long haul, even if no one else was. And that's how they wound up on both sides of the camera in the greatest Hollywood film ever made in a Sydney garage.