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 highlights Sadie's optimistic nature as she excitedly anticipates the event and imagines the best outcome. In contrast, Marco's response reflects his skeptical and brooding disposition, as he focuses on potential negative outcomes. This dialogue effectively illustrates the contrasting personalities of the two characters.