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 imagines a fun experience, while Marco's skeptical perspective is emphasized through his immediate doubt and pessimism about the situation. This contrast effectively highlights their differing outlooks on life.