The dialogue that best helps develop both characters for readers is:
“No way!” Sadie beamed. “Clear your schedule. We’re going to have a blast.”
“I don’t know,” Marco said slowly. “Are you sure we’ll even get tickets? This is going to be a pretty big deal.”
This dialogue showcases Sadie's optimism and excitement about the concert, illustrating her tendency to find joy in situations. It also highlights Marco's skeptical nature, as he doubts the likelihood of getting tickets and expresses concerns about the situation. Together, this exchange encapsulates their contrasting personalities effectively.