eugyppius

Two sources, including Theophanes (one of our major repositories of 7th-century Byzantine narrative history, you'll remember) say that Greek Fire was invented by an architect named Callinicus from Heliopolis.

For a long time, scholars have had differing theories of Greek Fire. There was the belief that it was some kind of substance that was thrown and exploded upon contact with water, or enemy ships. Perhaps even some kind proto-gunpowder.

More probably, it was naphtha or just crude oil, some sort of petroleum substance that could be burned and that would float on water. There are sporadic attestations in ancient sources, for the use naphtha in sieges, so this seems the most reasonable solution.

The idea that Greek Fire was a specific, secret formula, goes back to popular opinions in the medieval west, and also apparently in some less informed Byzantine sources. In fact, there's no reason to believe there was anything special about Greek Fire itself.

What Callinicus (if we believe the attribution) likely invented in the 670s was not the substance, but the flamethrower-type mechanism for projecting it. Theophanes says it was projected via a siphon, i.e. a pump of some kind.

From the illustration above, we can see that it was then forced through a kind of brass nozzle or tube, and thrown at enemy ships. The innovation was therefore this crude flamethrower device, which involved some heated chamber full of oil ...

Alternatively, I guess, you could just cover your enemy in unignited oil and then fire flaming arrows at him. Although this entire process seems highly dangerous, not a single accident is reported anywhere in the sources.