Actor/actress
If being yourself was really all you could do, there would be no actors and actresses, and there would certainly be no actors/actresses, or imitators telling other people to be themselves.
An actor/actress knows how to pose as other people, they do it all the time. The reality is you can be yourself, and you can also be a poser your entire life, and plenty of the people telling other people to be themselves are busy being posers themselves.