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 effectively showcases Sadie's optimistic nature and enthusiasm for the event, while Marco's response highlights his skeptical and brooding character. It clearly contrasts their differing outlooks and deepens readers' understanding of each character's personality.