Start Vague

I have always recommended to writers that they start out by creating and thoroughly understanding a world. I still think this is a good idea, but there is also a merit in doing the exact opposite.

Start by creating two generic characters. These are blank slates that you fill in as you feel like it. Have them start talking back and forth. It can be about anything. These characters are not fully-developed people yet, so nothing they say can be wrong or disagree with their characteristics.

As the conversation goes on, you glean information on who they are, or at least who they might be. If one person keeps having snappy comments and telling jokes all the time, you may decide that the person is a professional comedian. If you want it to be permanent, then you have it come up at some point, which canonizes it.

This method of starting vague has a lot of positive aspects to it. For one thing, you can start writing right away. And since most people get most of their ideas in the midst of writing, this allows the most freedom and creativity without needing revision. There are also some things that we just can't imagine will be important. A favorite color will rarely be of any significance, so it isn't useful to put in a character bio. But that doesn't mean it will never come up. If you start vague, then you can decide what your character's favorite color is if there ever comes a time.

Now, as your character amasses traits, there become fewer possibilities for them. For example, if a character is an adventurer, or some kind of wandering warrior, they are not likely to have any family ties and probably don't care very much about families. Since it is not a part of their world, it is something they will have no feelings about (unless their family was slaughtered or something, in which case they would have extremely powerful feelings). As such, once you have a decent idea of who your character is, you have to figure out which of the possibilities make sense to fill in the remaining gaps. It's like creating your universe in reverse.

Like I said earlier, both methods are credible and effective. The only factor is you. Some people write as actors. They become their characters and say what their characters would say. In that case, it is very important to have a firm grasp on who your characters are. Other people simply want to write a situation or a story. In that case, most aspects of a character are completely unnecessary, so the best thing to do is paint in broad strokes and fill in the blanks as need be.