When I interact with people, I think I tend to be in 'engaging' mode so will mirror them or I'll obviously smile if I think something is funny, or whatever. Pretty normal I'd say.
I also tend to show - pretty obviously - when I'm happy, or excited - so will have a big grin on my face and my face will become animated/expressive.
Also, re. photos, I've always smiled in them and don't feel fake doing so. (usually when I'm in a situation where I'm having my photo taken, I AM happy, haha)
But when I'm on my own, or at work, or am with someone and we're not actively talking, or I'm thinking... my neutral face is definitely read as 'sad', or 'what's wrong', or 'tired', or any number of more negative things.