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 response effectively showcases Sadie's optimistic nature as she enthusiastically lists all the fun possibilities, while Marco's skeptical character is highlighted through his pessimistic response about the event potentially being canceled and the day going badly. This contrast between their personalities is clear and helps to illustrate their character dynamics.