I will comment on this...
I personally smile if the band is killing it, if one of the guys plays something that MADE me smile, if the onstage banter
between me and one of the guys onstage tickled me... If someone in the audience did something that made me laugh, OR smile.
I DON'T just start smiling because I'm stood in front of an audience. If I saw someone just smiling at a show from beginning to end, I wouldn't think it was showmanship, I'd be thinking ... "Hmm, possible mental illness more than likely, or he's getting a LOT of money!" (Maybe even the recipient, or soon to be recipient, of a sexual favor

)
If you're on the stage a lot, or know you DESERVE to be onstage, then smiling isn't always a given... and I don't do weddings or similar gigs. I act onstage the same as offstage. Sometimes you laugh, sometimes you cry... Especially if the drummer is bad...
Hmm... I once did a recording session and finished the song in 15 minutes. I think I got between $5000.00 and $7000.00 for that session. THAT MADE ME ******g SMILE, BIG TIME!!!