I was in a drama class and one of the exercises was to convince a classmate that you felt some way towards them, e.g. "I love you". I recall varying my tone of voice, but I don't recall that I varied my expression. I remember it took forever to convince my classmates, which seemed odd to me - surely my tone was conveying the feeling. Thankfully, I was more convincing (natural) during productions.
I my have practiced in front of a mirror a few times, but didn't like it. Self esteem issues.
Yesterday my 10yo daughter was frustrated some classmates called her a liar and she observed that she was smiling (nervously) the whole time which probably didn't help her cause. She was not lying of course. Misunderstanding.
There's an Among Us live-action YouTube video which just tickles me pink. They ask a character if she's the villain and she says "no". They ask her "Then why are you smiling?" She says "Because any time somebody asks me if I am the villain, I smile. I can't help it and it's becoming a real problem." They say "If you're not the villain, stop smiling". But she doesn't and they vote her out of the game. Come to find out that of course she wasn't the villain.
Perhaps my daughter and that character need more practice in front of a mirror. 