Final answer:
Darcy surprises Elizabeth during her stay with the Collinses with his unexpected marriage proposal and his past actions that had interfered with her sister's romance, highlighting both characters' dynamic nature.
Step-by-step explanation:
During Elizabeth's stay with the Collinses, one of the things that Darcy does which surprises Elizabeth is his unexpected proposal of marriage. This proposal is shocking to Elizabeth because Darcy had previously expressed disapproval of her family and social standing. Additionally, Darcy's revelation of his interference in the romantic relationship between Elizabeth's sister, Jane, and his friend Mr. Bingley, as well as his continued treatment and perspective of Elizabeth's family, are profoundly startling to her. The complexity of Darcy's character, and the unexpected turns in his behavior towards Elizabeth, demonstrate the dynamic nature of both characters and are key drivers of the plot in Jane Austen's novel.