The dialogue that would best help develop both characters for readers is:
“We’re going to have so much fun,” Sadie said. “I bet we’ll get to meet the band. I bet we’ll even get to hang out with them!”
“We probably won’t even get to go,” Marco said. “I bet the whole show will get canceled and we’ll have a terrible day.”
This exchange clearly illustrates Sadie's optimistic nature, as she is excited about the potential experience and imagines positive outcomes. In contrast, Marco's response reflects his brooding and skeptical personality, focusing on negative possibilities instead. Together, these lines effectively showcase the contrasting attitudes of the two characters.