The dialogue that would best help develop both characters for readers, highlighting Marco's brooding skepticism and Sadie's optimism, is:
“I hope my uncle can get us tickets,” Sadie said. “I know I’m an optimist, but it could be really hard because the band is so popular.”
“Well, I’m usually skeptical and I still think we have a good chance,” Marco said.
This exchange clearly showcases Sadie's optimistic mindset, acknowledging her tendency to look on the bright side while also recognizing potential challenges. Marco's response reflects his skeptical nature but also shows a glimmer of hope, indicating that he might be open to the possibility of a positive outcome despite his usual reservations. This dynamic between them develops their characters well in the context of their friendship.