“When I taught creative writing, I would have my students do an exercise where they had to pick one of their close friends and write about the first time they met them. What were the writer’s first impressions of the friend? What was their initial dynamic with the friend? Then I would have them write a couple of scenes set in subsequent years showing how their impressions changed as they got to know the person better. Were their initial impressions correct? What did they learn that deepened their understanding of their friend? Showing that kind of evolution in perception and in the dynamic between two people is one of my favorite things about characterization.”
That's Sarah Stewart Taylor on Crimereads where she and several other writers weigh in on “the writing life”.