Today, the cameo feels like it's part and parcel of every notable movie release.
From Bill Murray in Zombieland to Stan Lee in a Marvel picture, these guest appearances are usually quite simple to spot, with their purpose to be easily visible to the audience for the sake of an in-joke, reference or gag.
But unlike the actors or regular media personalities (like Lee) who are often the focus of these cameos, the people who are usually behind the camera - when they do appear in front of it - are harder to spot.
Alfred Hitchcock was famous for doing this, with the director having a blink-and-you'll-miss-it role in most of his movies, and Hollywood has a long history of these behind-the-scenes masterminds finding their way into a shot - whether in their own movies or in movies made by their peers.
In fact, there's probably a moment like this in a movie you love (and you never even realised it), and sometimes, they're literally impossible to spot without first being told what to look for.
To that end, here are ten times famous movie directors popped up onscreen... and you may not have noticed.
10. Michael Bay In Mystery Men
Michael Bay's directing career was in the best shape it's ever been in back in 1999 - the critically mauled Pearl Harbor and Transformers were yet to go behind cameras and the man currently had the well-received trio of Armageddon, The Rock and Bad Boys under his belt.
Bay was also friends with fellow director Kinka Usher, whose first feature film, Mystery Men, was set to hit UK cinemas in December of 1999.
This friendship won Bay a small cameo role in the film, which, at the time, went unnoticed by pretty much everyone watching, since the explosion master wasn't as recognisable a face (nor as famous a name) as he is now.
He can be seen at the bottom of the frame as a frat-pack-leader who interrupts Geoffrey Rush's Casanova Frankenstein with the immortal line, "Dude, can we bring the brewskis?"