The core of the advice comes down to four questions you ask yourself - and answer - to learn about the character. There are other details you can add, sure, but this gives you a solid core while also giving you potential story for the character. Those four questions are:
- What is the Character's Name?
- What is the Character's Race/Job (other vital details can be here as well)
- What does the Character want?
- What does the Character fear?
Now, question 1 and 2 are pretty standard. Character's should have names, and they should have basic details about them. A gnome cleric is not a half orc paladin or a goliath rogue. Together though, these give you a reference point for a character and what they are.
Questions 3 and 4 are the fun ones. Why? Because these are the story questions. Now you can have longer answers, or multiple things, but the idea here isn't to answer what the character wants right now (that is more a question to ask in a scene) but what is it they're looking for in life, our out of life? What the character fears is not only just a way of giving them a weakness, but makes a good note for what they should be confronted with over a long enough timeline.
Now, for both GMs and Players you should be able to answer these questions about your prominent characters. If you can't, maybe think on it and figure out why you can't. Also, maybe find a way to answer it. It not only adds depth to the character, but it also gives direction and potential for future growth and change.
Some examples from my own games (as both a player and GM) could look like.
Name: Faida O'Broin
Class/Role: Tarish Mercenary turned Village Drunk
Wants: Her sister in law Brighid to be happy/safe, and for the nightmares of her past to go away
Name: Mirumoto Rei
Race/Class: Mirumoto Bushi
Wants: To win. To be loved. To do the right thing.
Fears: Not being good enough, and becoming the things she fights against.
Name: Inquisitor Rex
Race/Class: Human Inquisitor
Wants: POWER! (caps necessary)
Fears: Vader. The Emperor. Inquisitor Domina.
With all three of those you can get an idea for who the character is and what their story is just from what they want, and what they fear. The best part is, it's open enough for interpretation so three people could come up with three different ways of playing each of these characters, but still solid enough that it will remind you of what your idea was when you come back to the character.
Give it a shot. Maybe it will make things easier for you.