I'm back! So now we're on schedule again.
Dialogue: When they're at the dinner table and Katniss brings up that she knows the Avox, the conversation is straight-forward. Haymitch says, Effie says, not much frills involved. This keeps the story moving, because the words are more important than the actions at this point. We've already been situated in the fact that they're having a meal together. Whereas when Katniss has a conversation with Peeta later on, the way the two of them are interacting is important. Collins is sure to describe that Peeta is "conversational" as he mentions Gale and that Katniss is "observing" him. All of these descriptions to add to the dialogue gives a clearer understanding of their relationship.
World-building: Collins continually builds up the structure of this society, and in this case it's the Avoxes. This detail adds to how grotesque and brutal the Capitol is, even to its own citizens. She fleshes out the layers of their world through these people, who are not the Effie's and the prep teams like we've seen before.
Relationships: I think that this bit with Katniss and Peeta is important to start their involvement with each other. Peeta covers for Katniss, and in turn she tells him something a bit more personal. She's careful about deciding whether or not to share with Peeta, but it opens a way for her to at least see him as someone she can talk to. It's small, but it's an important development in the relationship they have.
What to learn in a nutshell: Keep conversation moving when the setting and people are established. But be sure to describe the characters and actions when it builds their personalities, situations, relationships, etc. Always be on the look-out to give perspectives of the world from all people's lives in the world. When building a new relationship, start with something small to make what grows out of it organic.