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 illustrates Sadie's optimistic nature as she enthusiastically looks forward to the event and imagines the best possible outcomes. In contrast, Marco’s skeptical response highlights his brooding character and tendency to expect the worst, reinforcing their contrasting outlooks on life.