It is always somewhat dangerous to set up ready-made categories and then apply them to something as various as a work of art, but certain definitions can help us to a clearer understanding of the characters we meet in Adam Bede.
A flat character is a one-sided figure, a character who exhibits only one or two human traits, usually in exaggerated form. Such a character's speeches and actions are never very surprising because they always spring from the same motivations and preoccupations, and he normally does not change at all in the course of the book. An example in Adam Bede is Mr. Casson, the innkeeper. Mr. Casson is very much impressed with his own importance, and whenever he appears in the novel, he is asserting or defending his dignity. He is a man with an inflated sense of his own importance, and that is all he is. In the same way, Mr. Craig, the gardener at the Chase and another of Hetty's admirers, is a know-it-all, and whenever we meet him he is dispensing (often false) information. Real people are never as simple as figures like these. The characterizations are superficial, static, "flat."
Round characters, on the contrary, possess the complexity which is the norm in real life. They are flexible and change in response to changed circumstances. Adam, for example, is capable of being harsh, gentle, loving, cruel, violent, shy, and so on; he has not one trait but many. And he learns a great deal in the course of the novel and changes gradually from a rather brash and immature youth to a self-disciplined and emotionally stable man. Adam is a "round" character, a fully developed and plausibly human figure.
A central character is one who plays a major part in the story and has a hand in the shaping of events. Central characters do meaningful things and have meaningful things done to them. A background character is normally not "on stage" very much, at least in comparison with the central characters. He can serve many purposes: he can help create atmosphere, as Wiry Ben and the other townspeople do; he can provide comic relief, as the men at the harvest supper do; he can provide incident, as Molly does when she drops the ale jug. But straight background characters do not affect the plot line in any very significant way; the drama moves around them, but it never really touches them.
The novel is so set up that the characters fall into three ranks depending on how directly involved they are in the novel's central conflict, the seduction of Hetty and its repercussions. In the "inner circle" stand Adam, Dinah, Arthur, and Hetty. These four are flanked by characters who are deeply affected by Hetty's seduction but whose lives are not changed by it: Mr. Irwine, Lisbeth, Seth, the Poysers, and Bartle Massey. Outside of them are ranged the vast host of straight background figures, people who exist on the periphery of the action.
It is easy to see how, with one great exception; the relative fullness with which each character is drawn roughly matches his importance to the story as a whole. All the characters in the third of our categories are "flat," while those in the second are more extensively developed and three of the four in the inner circle are presented completely "in the round."
This device is primarily practical. If each character were developed fully, the novel would become unbearably long. But at the same time, the principal characters must be presented as completely plausible human beings if the conflict through which they struggle is to have any meaning. So the relatively unimportant figures are merely sketched in, while many pages are devoted to the elaboration and analysis of the members of the inner circle.
The device also has organizational value. The reader will obviously tend to focus on those characters he knows most about, just as he would pay most attention to one close friend in a group of ten people. By setting up her characters the way she does, George Eliot leads us to fix our attention on the central issue of the novel.
The great exception to this scheme is, of course, Dinah; her characterization is widely considered to be one of the novel's major flaws. Although Dinah plays a central role in Adam Bede, she is clearly a straw figure, a plaster saint who can do no wrong. George Eliot puts her through some slight agitation and a change of heart toward the end of the book, but her basic view of reality does not change, as Adam's, Hetty's, and Arthur's do. She remains at the finish what she was at the start: a serene young woman, absolutely and totally devoted to duty, whose too-conscious piety tends to become cloying.