Have you ever had to act a specific way in your life? You probably have.
Many times, you have to act happy, when you're sad, because you have to be a "good sport", for example on a text/exam you got a bad mark on, a soccer game you lost, etc.
For some people, it's like that every day, so it is possible for people to act, instead of be themselves.
That sh*t sells albums but let's face it: People who "wear masks to hide their true selves" are just emos who think they are edge enough to have depth; in reality they are shallow cesspools of humanity who have nothing worth "uncovering" to begin with and even less to hide.
Honestly, I am who I say I am because I'm the same, inside and out. I do not have different personalities that depend on who I am with at the time. What you see is what you get. There's no reason for me to be anything other than myself, and that's that.