There's a wonderful list of writing rules by Kurt Vonnegut, of which one of them is: "Every character should want something, even if it is only a glass of water."
I remember reading that and something just clicking in my head. Up until that point, I'd bemoaned what I saw as the obsessive emphasis on goal orientated characters in writing advice. I'd think "well, plenty of people don't have clearly defined goals, and are pretty much just trying to figure life out".
What I'd neglected is the "if only a glass of water" part. Every person wants something, and every novel is to some extent about what its characters want. Even something like Invisible Cities (which, in case you don't know, largely consists of descriptions of fictional places) has Marco Polo and The Khan, who are people, and who therefore want things.
So, my answer is to make your characters want something. Giving them an agenda of some kind doesn't mean turning the discussions into heated arguments, it just means making them human beings. Maybe their agendas are as simple as character X wanting to seem a certain way to character Y, and character Y wanting to seem a certain way to character X. Allow this to play out while the debate occurs, and the reader will be left with the impression that they're hearing real people speaking (rather than just the author using fictional people to push through a viewpoint).
For a nice, famous example, I think there's some good examples of this in David Foster Wallace's work. There are quite a lot of debates and discussions and monologues that apparently do very little to further the plot, but do a great deal to further character and theme.
Note that you'll have a little more slack on the other stuff if the debate itself is fascinating. Another of Vonnegut's rules is "Use the time of a total stranger in such a way that he or she will not feel the time was wasted." Obviously, this is the most important rule of all (and probably the only one that should never be broken).