Characteristics of Round characters:
Round characters are multidimensional, revealing contradictions and complexities as a part of their human nature. They tend to be fully developed with dynamic qualities and have ability to surprise the audience. Mostly, major or main characters are round characters.
Characteristics of Flat Characters:
On the other hand, the flat characters lack depth and remain same over the course of a narrative. Thus, they are one-dimensional. They do not surprise their readers, as they have no issues to solve; therefore, many blockbuster movies use several flat characters to avoid complexities. Simply, they are as flat as a bottle of cold drink, which you might have left on your kitchen cabinet last week.