Arguments are another matter, as long as they have a bearing on events of the plot, and so are thrashing out their opposing views in a manner that foreshadows their decisions and acts -- or the acts and decisions of one of them, with a minor character acting as a foil and then vanishing.
Theoretically, this would be just fine for mere discussion, though it does have the problem of needing drama, but merely discussing the underlying principles of the story raises the grave danger that you put them in the mouths rather than the actions of the characters. Which is also a danger when you have characters argue about them, though less of one. Two characters can fiercely argue about whether to keep a promise that does not actually entail doing something wrong and then, in fact, never face such a promise, though it seems to me to happen less often.
Perhaps it's just that better writers make them argue to produce conflict and then remember to tie up the thread by having it significant -- or introduce the argument to foreshadow a conflict that's later in story time.
