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 and excitement about the upcoming concert, while Marco's skeptical response highlights his brooding and cautious personality. Their contrasting perspectives are effectively showcased, helping readers understand their characters better.