You can be taken seriously even if you indiscriminately smile out loud.
This is all about your “game face.” Keep an awake, alert, alive look that invites people to talk to you. Why? Because you need people to talk to you with suggestions, problems, advice, watch-outs, issues, mistakes, concerns, opportunities, fears, and frustrations. If they don’t talk to you because you present yourself as closed, shut off, uninterested, bored, confrontational, and downright scary, you are the one who loses. And when I say lose, I mean that people won’t interpret you correctly. They might mistakenly assume your intent, and you may not get credit for your true thoughts. As mistaken as their impression of you may be, it might cause them to react differently from what you expected or wanted in the communication exchange. And it was your fault for not taking responsibility for making sure that your actions were consistent with the message you intended.
An open face, especially sporting an easy smile, makes you look approachable and approving of others—“You’re okay; I’m okay.” You give people a lift and convey the impression that things are sinking in, not bouncing off of you. An expressionless flat face or a smirk, sneer, or even slightly pursed lips says, “You are not okay; I’m okay.” Besides, if you keep at it, as you get older, you’ll get mean lines around your mouth, and you’ll scare your grandchildren away.
Benton and Wright, co-authors of The Leadership Mind Switch (McGraw-Hill, 2017)