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 exchange shows Sadie's optimism and excitement about the concert, which aligns with her character description as someone who looks for the good in every situation. Marco's response reflects his skeptical nature and concern about the logistics of getting tickets, highlighting his brooding tendency. This interaction effectively contrasts their personalities and gives readers insight into their dynamics.