You'll run across authors who do all sorts of things, filling out a form; literally interview their characters, "What's your strongest childhood memory?" "How does character X annoy you, and what do you most like about them?"; or one I do at times, free writing involving the character as a form of exploratory writing. With a current project I have three characters that are college students, for whom I knew almost nothing beyond that (okay, two are for lack of a better word aliens from the other side of an interworld portal, one of a handful that opened on Earth in the early 20th century, and the third a human whose father takes him to the other side of the portal when the company he worked for transferred him there). So I free wrote short snippets of the characters when they were younger to get an idea of who they were. When I do this I give myself permission to just go with the first thing that comes to mind. It's purely exploratory and if I decide something isn't right I can just delete it.
But the value of these is to get an idea of the characters' goals, desires, and possibly their fears. Then when you have a scene you can ask, "Okay, because of their older sister's scandal they don't handle anything that smacks of cheating or betrayal well. A month ago they and their long time companion woke up hung over with a third person in their bed. That third person has just shown up at the front door saying they're pregnant and have been thrown out of their home. How would they react to this?" The more of a feeling you get for the character the less you may need to consciously ask that. But when you're still figuring out how the character ticks it can be helpful to have worked out those desires and fears. If you're taking the time to consciously ask the, "Okay, what do they think about this?" don't stop with the first answer. "I'd like to slam to door in the character's face, how dare they show up again. On the other hand the character might well be pregnant with my kid. And even if they I mean to they got someone younger drunk and pulled them off to bed, mom and dad are going to kill me and if I don't treat the poor kid (college freshman, they're all naive kids) right mom and dad will kill me a second time!" Partly because the first thing that comes to mind won't necessarily be the best idea but also partly because people can be complicated and you might decide two or more of those answers influence how the character reacts.
It might be helpful to explore different systems of archetypes (whether Myers Briggs types, or Jungian types, or whatever) and what the stereotypical views of each are and how they're supposed to interact with each other. That would give you a framework to use. So you could more or less say, "Okay, character A is a Jester, B is a Ruler, and C is an Innocent..." The archetype can be like the outermost layer of a Russian nesting doll. The most easily seen part of the character. But the other details you come up with for the character by asking and answering questions (however you chose to do that) become the other layers of doll within that make them different from other examples of that archetype.
You might try looking at established sets of characters and ask what would happen if you put them in different situations. Not necessarily writing an actual fanfic, just asking, "Okay, each of the characters in Stargate SG1 wakes up finding a dead body in their room. What do they do? Why?What do they do that's the same as each other, what's different? What's something each character would notice that's different from the others?" Or the crew of the Serenity, or Leverage or White Collar or jump back a few decades and ask it about the cast of the A Team or Hogan's Heroes. Then try asking the same about your characters. And experiment with different things. What works for one person won't necessarily work for you. It doesn't mean you're wrong, it means that tool doesn't fit your needs at this time. It might be useful later, it might not, just keep looking and trying.
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Will the transhumanist future have catgirls? Does Japan still exist? Well, there is your answer.
But the value of these is to get an idea of the characters' goals, desires, and possibly their fears. Then when you have a scene you can ask, "Okay, because of their older sister's scandal they don't handle anything that smacks of cheating or betrayal well. A month ago they and their long time companion woke up hung over with a third person in their bed. That third person has just shown up at the front door saying they're pregnant and have been thrown out of their home. How would they react to this?" The more of a feeling you get for the character the less you may need to consciously ask that. But when you're still figuring out how the character ticks it can be helpful to have worked out those desires and fears. If you're taking the time to consciously ask the, "Okay, what do they think about this?" don't stop with the first answer. "I'd like to slam to door in the character's face, how dare they show up again. On the other hand the character might well be pregnant with my kid. And even if they I mean to they got someone younger drunk and pulled them off to bed, mom and dad are going to kill me and if I don't treat the poor kid (college freshman, they're all naive kids) right mom and dad will kill me a second time!" Partly because the first thing that comes to mind won't necessarily be the best idea but also partly because people can be complicated and you might decide two or more of those answers influence how the character reacts.
It might be helpful to explore different systems of archetypes (whether Myers Briggs types, or Jungian types, or whatever) and what the stereotypical views of each are and how they're supposed to interact with each other. That would give you a framework to use. So you could more or less say, "Okay, character A is a Jester, B is a Ruler, and C is an Innocent..." The archetype can be like the outermost layer of a Russian nesting doll. The most easily seen part of the character. But the other details you come up with for the character by asking and answering questions (however you chose to do that) become the other layers of doll within that make them different from other examples of that archetype.
You might try looking at established sets of characters and ask what would happen if you put them in different situations. Not necessarily writing an actual fanfic, just asking, "Okay, each of the characters in Stargate SG1 wakes up finding a dead body in their room. What do they do? Why?What do they do that's the same as each other, what's different? What's something each character would notice that's different from the others?" Or the crew of the Serenity, or Leverage or White Collar or jump back a few decades and ask it about the cast of the A Team or Hogan's Heroes. Then try asking the same about your characters. And experiment with different things. What works for one person won't necessarily work for you. It doesn't mean you're wrong, it means that tool doesn't fit your needs at this time. It might be useful later, it might not, just keep looking and trying.
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Will the transhumanist future have catgirls? Does Japan still exist? Well, there is your answer.