It kind of depends. The dynamics of book clubs vary a lot.

Hopefully there's going to be a facilitator of some kind. They usually prepare some questions to help get everyone talking. My experience with library groups is usually someone from the library handles this.
If it's a fiction book things are usually a lot looser. You talk more about impressions, feelings, themes, likes and dislikes, things people appreciated about the characters, etc.

Non-fiction will usually be more about what people learned, how it compares with other things people have read on the topic or differences from preconceptions.

Usually there's a few people who are more likely to talk. With a library group I'm guessing there's a couple older people who go frequently and already know each other. They'll probably be more talkative.

But it's about as low stakes a social thing as you can get. You've got a topic, a mutual interest, and someone will almost certainly show up not having read the book, so you'll be more prepared than they are.

Depending on how social your group is or other connections you can go off on a lot of tangents.