Give your characters compelling dialogue. Strong dialogue tells your readers exactly who your characters are within moments of them speaking their first lines. An immense amount of information can be conveyed with dialogue, without even saying anything specific.
Well written dialogue is comprised of something as simple as a character buying groceries. Even in a fantasy novel, we need to be able to relate to the characters and understand who they are on a raw, visceral level, and dialogue is the vehicle for achieving this.
But keep in mind that compelling dialogue doesn’t necessarily mean having lots of dialogue. You may have a character that says very little, which in turn says a lot about them without the use of words. You may have one character that’s speech is filled with slang and another that sounds like a preacher.
Having characters that speak in their own voices in tremendously important, and make sure that each of your characters sounds sound different, having their own voice.