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 nature, as she excitedly dreams about the possibilities of the concert, while Marco's response reflects his brooding and skeptical character. It shows the contrast between their personalities and sets up their dynamic well.