How does Eliot develop multiple characters at the same time within the limited narrative structure of the novel? Using Fred Vincy as an example.

The narrative structure of the novel is limited: within each chapter only so many plot events can be told and a handful of characters developed. Within the few chapters that make up this week’s reading, Eliot manages to flesh out multiple characters like Fred, Dorothea, Lydgate, Rosamond etc. How does she manage to do this, when she generally focuses on one to two main characters per chapter?

Not that this inward amazement of Dorothea's was anything very exceptional: many souls in their young nudity are tumbled out among incongruities and left to "find their feet" among them, while their elders go about their business. Nor can I suppose that when Mrs. Casaubon is discovered in a fit of weeping six weeks after her wedding, the situation will be regarded as tragic.