One way you can do this is to use dialogue and speech mannerisms to quickly identify their personality. what slang do they use? How do they react to things?

Basically, it's easier to think of your characters as THOUGH they were real people, not necessarily a DnD character sheet. Think, "well, why does Hero McSlaysALot want to kill the dragon? Ok, he wants the princess, but why is he doing it beyond that? Is it out of the kindness of his heart overall, or for more selfish reasons?" etc., and then whenever you get a chance to let your character's inner voice and mannerisms shine through, you let it.

Using other people you have met (including the mannerisms you yourself have) is another great way to flesh out a character and make them real, because you already met this person and know how they react to certain events. Just adapt them to the situation at hand, and work from there.

If the character is realistic and sympathetic, or at least understandable and coherent, as long as you let their true self shine through on occasion through dialogue and action (without revealing everything about them), your reader will be able to get behind them.

This, of course, ties in to "show don't tell" quite a bit, letting your characters breathe and go at their own pace. This can be hard if you really have a passion for these characters, but if you let them progress and turn into lifelike representations of themselves, and use fight scenes or internal conflict to intrigue the audience in the meantime, you will have a much better chance of striking the right chord with your audience.

Hopefully this wasn't too dull for you to read through, I really need to condense my words down in these posts. Thanks for reading and commenting, and I hope you found this useful!