The dialogue that best helps 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 showcases Sadie's optimism as she enthusiastically predicts a positive outcome, while Marco's response highlights his skepticism and tendency to expect the worst. Their contrasting views are effectively illustrated, giving readers a deeper understanding of each character's personality.