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 effectively showcases Sadie's optimism as she enthusiastically imagines the best-case scenario, while Marco's response highlights his skeptical nature and tendency to focus on the negative. Their contrasting outlooks are clearly presented, providing insight into their personalities and how they interact with one another.