Someone nervous before a first date, a job interview, or a difficult conversation gets told to "just be themselves," and the advice is almost never useful in the moment it's given. It's not wrong, exactly. It's incomplete in a way that makes it useless precisely when someone needs it most.
The advice quietly assumes a finished, legible self sitting there ready to be presented — as if the only obstacle were nerves getting in the way of showing it. For a lot of people, that's not the actual problem. The problem is not having a clear read on what "themselves" reliably looks like across different situations, which parts of their behavior are actually consistent and which shift depending on who's in the room, what they actually want out of the interaction versus what they think they're supposed to want. "Just be yourself" tells you to relax and trust a picture you don't have.
What the advice is actually reaching for
There's a real, useful idea buried in the cliché: performing a version of yourself you think the other person wants usually reads as exactly that — a performance — and it tends to backfire in ways that being straightforwardly, unglamorously accurate about yourself doesn't. That part is true and worth keeping. The failure is in assuming everyone already has ready access to the accurate version, when noticing your own patterns clearly enough to represent them honestly is a skill some people have done a lot of work to build and others haven't had much occasion to.
A more useful version
The more useful advice, if less quotable, is something closer to: know what a handful of your actual patterns are, specifically enough that you'd recognize them in yourself rather than just in a horoscope-style description, and let the accuracy of the interaction come from a real description rather than a performance either of "your best self" or of "an unfiltered self." Neither of those extremes needs faking, but both require having looked closely enough at your own behavior to know what's actually there.
That's a harder ask than "relax and be yourself," but it's the version of the advice that actually helps someone standing outside a job interview trying to figure out what to do with their hands.