It's easy for me. But, growing up, I trained in dance classes and performed in dance recitals. They teach you not just to plaster on a smile, but to find meaning in the music, feel the feelings behind it, and then let that inform your face and body in order to tell the story. This was not natural for me, but I was taught how and it became OK.
So now, I still have "performance mode", and I can be very expressive and presentational. I can respond in all the "right" ways as long as I know that I am supposed to. For example, If I am going to a party and expecting to meet some new fancy people, I know to turn "performance mode" on and I can be genuine (fake) smiley up the wazoo. It's just acting.
However, in normal day to day life, it's way too exhausting to be acting all the time, so "performance mode" is off and I tend lack facial expression in general. I wouldn't be able to fake smile during casual small talk that I'm not prepared for.