What is a word for someone or something that superficially seems to have a lot of substance/character but is actually shallow/empty? I can't help but think I've heard a word like this before. "Glib" comes close but seems to speak more to words rather than character.
There's also phony in the sense of: 5. an insincere or affected person.
You might also use a number of the synonyms listed under 2. One who fakes: in the thesaurus section of the same definition.
A colorful phrase for such a person, in U.S. English, is "all hat and no cattle." The literal reference is someone who dresses and maybe acts like a cowboy (with a fancy ten-gallon hat, for example), but knows nothing about ranching, and is used more generally to describe someone who is all show and no substance.
If you're looking for a single word, here are a couple of options:
Perhaps a proverb rather than a word will do.
empty vessels make most noise (or sound)
Those with least wisdom or knowledge are always the most talkative. — ODD
Essentially this proverb is the antoymn of Still waters run deep.