a round character is a character that makes progress as the story progresses or head to its climax while a flat character he's a character that is stagnant is a dominant character
for example a round character perhaps who started as a poor farmer but at the conclusion of the story the Farmer becomes rich and wealthy
but a flat character is perhaps one who was a maid at the beginning of the book or story and at the conclusion of the story at the climax the person remains a maid it's as simple as that
Marnie out !