The best dialogue response that develops both characters for readers is:
“No way!” Sadie beamed. “Clear your schedule. We’re going to have a blast.”
“I don’t know,” Marco said slowly. “Are you sure we’ll even get tickets? This is going to be a pretty big deal.”
This dialogue effectively showcases Sadie's optimistic personality as she expresses her excitement and enthusiasm about the concert, while also highlighting Marco's skeptical nature as he questions whether they will actually be able to get tickets. The contrasting reactions encapsulate their character traits well and illustrate their friendship dynamic.