There's similar information in this awesome thread by Shy, but I wanted to take a crack at this topic myself.
How do you make a character that will be consistently interesting to roleplay?
Lots of people getting into RP, and even many experienced roleplayers, find themselves making characters with personalities that don't have staying power. They end up not participating as much as they'd like "because it's what the character would do," or playing them and trying to keep their personal story and part in others' stories fresh becomes difficult, or you just lose interest after a short time. It happens to everyone. Especially me.
And from my experience of making characters that don't last I've learned to suss out at least part of the secret formula to make something original, interesting, and enjoyable to play. I can't guarantee this will work for you, but I gamble it will work for most players who give it a shot!
Step 1. Don't Write a Backstory
Not yet, at least! This is key. We need to start by forming a person as they are in the moment. If the person you will inhabit is not interesting to watch do what they do, then all your hard work creating a long and intense narrative will go to waste.
Step 2. What does this character want that they can't easily acquire?
The essence of stories is conflict, and conflict comes because characters want something and have to leave their comfort zone, the familiar, to find it. They may not even end up getting it, but the pursuit makes the story. The pursuit also immediately helps shape what the finished character will be. What kind of person wants this specific thing? What other kind of people could want it? Why's it so important? Which combination of answers to those three questions seems like it will be the most fun to play, and for others to interact with?
The key words of this, that need to be reinforced, is 'can't be easily acquired.' If your character has some terrible curse, and another character has a spell that ends curses, that's that. If your character wants to be the king, however, that requires work and the help of others- and that's where the magic of the experience of role-playing happens. The goal should absolutely not be achievable alone. It should also not have a guarantee of success after a predictable period of time.
Congratulations! You've just made a story arc that you and everyone you interact with will play out. Without a story arc you're engaged in, you're not playing an active character, and that's where the problems lay.
Step 3. What would this character never give up?
Since you've just created a lofty story arc, you now need to add baggage and vulnerability to make it interesting. The form this value takes says a lot about the true nature of the character. Is it an object? A place? An idea? A person? A habit? This should not be one and the same with the character's goal, as it's a seasoning to add risk.
With this, you're starting to create a formula of a plot arc. "This character wants X, and wants to protect Y while getting it." Guts wants to kill Griffith but wants to keep Casca safe from demons while he does it. Grom wants to conquer Azeroth but wants to keep his honor intact while he does it. Frodo wants to travel to Mount Doom but wants to keep the ring for himself while he does it.
The threat of losing what the character values serves as a call to action, to limit their options, to add stakes and depth, and most importantly creates the constant question of "what does the character want more, their goal or their value?" When that question requires an answer, you're playing through one of the most important parts of that character's life, and you'll never forget it.
Step 4. What would this character give up if they had to?
No character gets through their story unscathed, and asking what they're prepared to sacrifice for Step 2 and Step 3 lets you understand their worldview and morality. Some characters sacrifice their humanity to see the job done. Some sacrifice their lives (I don't recommend this if you want to keep playing them!). Some sacrifice comfort, some sacrifice their optimism, some sacrifice their pessimism. Some sacrifice trust, some sacrifice caution.
There's a million different factors that create a human heart, and knowing which are able to bend and which they'd eagerly break makes viewing the world through their eyes much easier. The value system of the character will dictate how they react to stressful and non-stressful circumstances, how they feel about other people, how dedicated they are to their goal, and how dedicated they are to their value.
And as these sacrifices of the character's former identity are made, the character changes and grows into something more realized and more interesting. This is character growth, and if you learn to embrace character growth, you will enjoy the character for as long as you enjoy the format you're playing them in!
Step 5. How does this character prefer to solve problems?
The point of this question is that there is no right answer. If the character prefers to use force, they're weak in situations that don't require it- or that would be actively undermined by it. If the character prefers to plan extensively, they're weak in situations that happen by surprise and have a shortened timetable. If they prefer to enlist the aid of others, they're weak in situations where they're caught up on their own. And if they're bad at their preferred method, that's all the more interesting: it means you get to witness them improve, or at least compensate, to become more effective at pursuing their goal.
Naruto is a badass ninja who started out awful but he prefers to talk problems out with his opponent. Anakin is a powerful Jedi but he doesn't know how to deal with people or structure. Night Owl has a huge suite of gadgets and a lot of martial arts training but he loses sight of the big picture and can get swept up in events he's powerless to stop. Don Quixote uses his martial prowess and chivalry to save the kingdom, but everyone else thinks he's crazy.
This fills in the 'how' of what this character would be like to play. There's just one more step to bring it all together.
Step 6. What's unique about this character's way of acting?
This is where you make something memorable. A quirk, a deficiency, a delusion, a hypocrisy, a tic, a signature way of speaking, a strange hobby, a trivial obsession. These and more serve to flesh out a character's personality. The important part of this is that it should be apparent within the first day of meeting the character. Dark secrets that you'd only tell to your closest friend are worthless in creating intrigue compared to some distinguishing feature that leaves an impression.
With all of this you can then get an idea of what they'd be like to actually play in-game. Our final formula looks something like this:
The character, notable for *quirk*, is on a journey to achieve *goal.* They cherish *value*, and prefer to tackle challenges with *method,* which leaves them helpless when *opposite of method.* On their journey, they may have to give up *sacrifice* to make their dreams come true.
To put this into practice, I'll use it on the character I've created just to engage with AoC:
Arquus Nevros, notable for being overly friendly with everyone he meets (including his enemies), is on a journey to collect or create a thousand stories of the return of civilization to Verra. He cherishes his tavern and the band of crazy adventurers that staff it and accompany him on outings to star in the greatest tales he can tell. He prefers to tackle problems through people, whether it be amassing allies or convincing enemies to leave him alone, but this leaves him powerless when alone and confronted with a threat greater than his own middling martial prowess. On his journey, he may have to sacrifice his desire for independence and neutrality if he's to keep his bar and his friends safe from those that would be the villains of his own tale.
This isn't a backstory; it's a character bible. This is everything I need to get in and start playing. I can flesh more out as I go along! And as I interact with and connect with other players' characters, what I have written here will inevitably not resemble the final product after a year of playing. If that's the case, then I've succeeded.
If you want an exercise for fun in the years we're going to be waiting for the game to drop, try making your own character paragraph in this thread!
Oh, and before I forget, the final, optional step:
Step 7. Write a Backstory If You Really Want
How did this person become the character described in the paragraph? You can come up with a comprehensive explanation, or just make decisions to explain things on the fly. If you do the latter well enough, no one will be able to tell, because no one's going to read your backstory post anyway. (Editor's note: Someone will probably read it.)