It is pretty much common sense in writing that a story needs unique and compelling characters to succeed. It shouldn´t be just talking heads, they should have depth, backstory and at the same time you don´t want to punch the audience in the face with the depth (“Look at that person and his great and compelling backstory”).
While there may exist some genius writers who don´t have to outline or structure their notes for the characters, most writers certainly do. And the common practice for a structured plot is good outlining and character development sheets.