What moves the plot forward? The actions and reactions of the characters certainly have a lot to do with it. They might be reacting to events beyond their control; a conspiracy, a catastrophe, and so forth. Or, they could be driving the story forward, as happens in Goodricke’s Time, by their own actions. In any case, it is their actions ought to be believable, they should remain ‘in character’. The reader must have a clear concept of the personality of each main character, something not as easy as it may sound.
The creation of the main characters has two major elements. First, you must have a conveyable concept of their basic personalities. This is critical. If you struggle for convincing adjectives to describe someone you know personally, then how well can you create a fictional character who behaves in a consistent manner? A ‘blueprint’ often suggested in aid of this is the sixteen personality types devised by Briggs Meyer. I did not think it appropriate to simply go down the list of these sixteen personality types ‘shopping’ for the one that seemed to fit. Rather it provides a baseline from which to operate by outlining characteristics of each personality type. For example, the INTP (introverted, intuitive, thinking, perceiving) types are those often described as quiet and analytical. They enjoy spending time alone, thinking about how things work, and coming up with solutions to problems. INTPs have a richer inner world and would rather focus their attention on their internal thoughts rather than the external world. In Goodricke’s Time this comes close to describing Goodricke himself. The usefulness of understanding each of your characters personalities helps them in a way, to push off in their own direction. One of these personality types will likely best apply to any character you have in mind. It can help to exercise this ability to describe personality types beyond the simple ‘introvert’ vs ‘extrovert’.
The second element is to keep in mind that they drive the plot. The main protagonist should be easily perceived as someone who will want something with all their heart. But there will be others who would thwart him. The reader must be able to care what happens to your main character. If they do not, then why turn the page?
Characters need not be wholly consistent throughout. They may self-evaluate and change their thinking. Goodricke, unlike Piggott, was not interested in women, in relationships. His life, his entire life, was science. But over time he began to rethink his priorities. Why this happens, the experiences that lead to this had to be carefully considered and placed within the narrative. His deafness played a large part in this reappraisal of his life’s path. Your main character will likely not have this complication but will still struggle in a different way.