Tang came into my head more or less fully formed, certainly to look at, as did Ben. With Tang, a number of his characteristics came about because of his physicality, such as the fact that he can’t cook because he’s too short and can’t reach the stove. He was also never exposed to much in the way of teaching from Bollinger, so he arrives in Ben’s garden underdeveloped save for the experiences he has had on his way over to England, which the reader never finds out about.
The journey that Tang went on to reach Ben also gave rise to the robot’s character from a visual point of view – he’s dirty and battered, which means other robots/androids look down on him even more than they would have before. This has given Tang a bit of a complex, and makes Ben’s kindness to him even more of a reason for the robot to latch onto him.
Tang’s appearance, being the only thing Ben really knows about Tang for a quite a while, is the foundation of their relationship. Ben is a broken man, though he doesn’t realise it, and feels an affinity for Tang who is in the same boat. They are both underdogs.
A large part of Ben’s character started out as a series of practical considerations. He is wealthy because it meant I didn’t have to get bogged down in the financial implications of a round-the-world trip, which might have detracted from the story I wanted to tell. But that in turn leads you to question why he is wealthy. For him to be an underdog he needed to have the odds stacked against him, which meant I had to put him through some tough times. He also needed a reason to be able to drop everything and head off on the journey with Tang, so he needed to have no ties at home.
So I took away his parents and had his wife leave him, in addition to being perpetually unemployed. These things solved the financial issue and the ties to home, but it gave him a serious backstory which needed to be addressed throughout the book.
In addition to the sense of ‘you and me against the world’, Ben and Tang’s relationship developed just by sending them off round the world and seeing how they would each respond to the situations they found themselves in, knowing the issues and limitations of both. Ben has a fear of being a father in case he is terrible at it, but by necessity taking care of Tang assures him that he could actually cut it as a dad, though it takes other people to tell him this.
So, to sum up, I think for me character + experience = story.