In previous entries we talked about conflict, using your experiences and the world at large to inspire you. But what if we have a decent start on a plot and know our conflict, our theme? Most likely the obstacle is your character(s).There is just about as much advice on this topic as there are characters in fiction, but some basic truths undergird the subject.
Characters need to be convincing. That doesn't need to mean that they can't make what the author would consider ridiculous decisions. John Steinbeck said that only when dialogue is spoken aloud does it become speech. And similarly, Joseph L. Mankiewicz said that "realist" dialogue doesn't exist, we are all actors. And that is how you should be considering your characters.
As we dig deeper into the character, why would he prefer to ask for Grey Poupon instead of roaring about the mustard? Were his parents educated and well employed people of high social status? Or did his parents leave him at the doorstep of an adoption center and he spent his life bouncing around through the state systems? What is the backstory?
That leads me to the second point.
We could go on for a long time like this, but the idea should hopefully be hitting home now. If your plot calls for Man to meet Woman and then betray her but go through a development and turn it all around in the end, we need to know why the man betrayed her. Sure, it could be because a gorgeous woman tricked him, but why was he able to be tricked? Did he simply fall for a beautiful woman because he was not used to that sort of attention?
The caveat to all of this is that as Guy de Maupassant said: "you must render, never report."
Like computer coding, a program comes together from a summation of all of its parts. There isn't a "holy grail" singularity that IS the program. Rather, it is a symphony of components working in unison. A character must be rendered through gradual construction of his pieces, not through a single gospel that reports it at once.
Don't bore the reader to death with explication for a character's decision or thought (which is a whole other article). Instead, let it come through in dialogue or description. The first paragraph of American Gods by Neil Gaiman says more about character than 2 pages of explanation.