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 highlights Sadie's optimistic perspective as she enthusiastically anticipates the possibilities of the concert, while it showcases Marco's skeptical nature as he immediately thinks of negative outcomes. This contrast effectively illustrates their differing personalities.