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 effectively showcases Sadie's optimism and excitement about the upcoming event, while also highlighting Marco's skepticism and caution regarding their chances of getting tickets. It clearly contrasts their personalities and sets the stage for their dynamic.