This happens to artists a lot. When drawing the emotion/expression they wish to convey, that same expression also comes out in their faces.
I do this when writing expressions.
I also laugh out loud at my own jokes. And I have a really maniacal laugh. A megalomaniacal laugh. A real, genuine, no-mister-Bond-I-expect-you-to-die evil EVIL laugh.
And, to add the icing to the cake, I sometimes mutter choice phraseology. Either to test how it tastes [words have flavour! It’s true. Some turns of phrase are absolutely delicious as they trip off the tongue] or, because I think I’m a literary genius and that thing I’m in the process of putting down is just too frigging brilliant to herald into the world with silence.
I have been unaware that I do most of these things[the muttering was hard to miss…] for years.
I have also been committing literature on public transit. Usually in palm-sized notebooks easily obtainable from dollar shops.
…no wonder I always had a seat to myself.
And it also explains why, in the midst of my wrestling with the muse, relatives have wandered in, looked at me and said, “Oh, you’re writing.” and vanished before I could surface back into reality to ask them what the hell they were talking about.
…also shoulder-surfing me is never a good idea, for reasons that quickly become obvious. Writers always need some new and interesting ways to knock some character off.