Preparation? Maybe. The right timing? Sometimes it is the way the sun is shining? This is a very hard question to answer as a GM.
I am going to hazard a guess and say that most people may go with this for an answer. It is hard to say if this is the case because my experience tells me that you can work on a character as much as you want and it can fall flat. It also of course depends on what good means to you? does good mean that it can stand up to a fight against a group or that it is memorable to the group. This question is full of vagaries but I will say good to me means the character is memorable to me, or feels right playing it.
For example, Day 3 of this challenge is a good character. Thibault felt just right and he was memorable. The players finally caught up with him and blew his head off but they got him. Thibault was a Machiavellian character in all aspects and his portrayal was just right. But preparation was something that Thibault did not have – my experience tells me that this is not the reason for making a good character.
As opposed to preparation, perhaps it is the portrayal of the character. Thibault was flamboyant and memorable with his mannerisms and speech patterns. But I am woeful when it comes to portrayal of these things, so maybe not this?
When I ran Thibault, I gave him 100%. I let the character attitude and positioning in the story guide me without a prepared framework. Attitude could be the thing here that gives a character an edge.
Thibault was short, rotund with a wiry mustache. he was certainly affluent, as his surroundings communicated but little else was given in the way of description. It would be interesting to see how others pictured him, and if the descriptions match my own mental images. I do not think description is the key.
Must be magic. Totally magic.
Actually, it is a little of all of these things – including magic. The balance is what you want to get right. The balance for every character is going to be different and so this is much more an art than it is a science. An art that is well worth attempting – but it is an art in and of itself. I wish you well in creating your own pieces of art! Keep rolling…