I had sworn to stick to acting when it comes to film, but to be honest I am a) not really a very versatile actor and b) I prefer other people doing the heavy lifting when it comes to creating roles.
For the first few years of directing film I made a point of not acting in my own work, because I felt, and still feel, that doing one job is hard enough; one of the two is bound to suffer, if you juggle the responsibility of guiding actors through a performance and at the same time serving as their scene partner. There is no way you can commit 100%.
I have made two exceptions thus far and whilst I got lucky enough for the material to work out, I have gone back to simply directing, if there is no one on set to guide me through my performance.
As a film director I slid into it just as accidentally as I did into producing, however, I was a producer before I started directing film. And if it hadn't been for my then-DoP, the whole thing would have probably looked like an open air stage production. But we made it through and several years, a few shorts and a feature film later, we are still here and still making films ... and they no longer aim for the Tony Award.