The dialogue that would best help 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 excitement, while simultaneously highlighting Marco's skeptical and brooding personality. It sets up a clear contrast between the two characters, emphasizing their differing viewpoints on the situation.