I always wondered if going off-topic in a dialogue is a bad or a normal thing.
For example, let's say I'm writing a novel where the main theme is love and loss. Is it bad if the characters start talking about their favorite breakfast, or a cousin they dislike? Or maybe about another theme like alienation?
Every time I find myself in this situation I get stuck. I don't know if I should removed the part, make it fit somehow with the theme or just leave it there.
In real life, conversations ramble, so it's unsurprising if your dialogues ramble as well. This is not necessarily a bad thing. In realistic literary novels, it would be unsurprising, or even expected, for your dialogues to include long tangents and unrelated content. This is part of that genre's attempt to present situations naturalistically. In other genres readers expect a tighter degree of focus, and so will be less tolerant of long conversations that don't appear to relate to the story.
However, just because a dialogue doesn't directly relate to the theme or plot doesn't mean that it actually accomplishes nothing. Dialogues can be used to set mood, to demonstrate character, or to illustrate the setting. So consider: does your characters' conversation actually contribute nothing to the story, or is it adding something a little more subtle?
If it distracts from your plot and from your theme, then yes - sooner or later, that dialogue should be rewritten or cut entirely.
Real conversations get sidetracked; they go off on tangents. If you do a lot of your writing by figuring out "what would they say next," you're likely to follow that natural tendency. The problem is, if it's really nothing but a random offshoot of conversation, then it probably won't interest the reader. It may confuse him, or he may find it boring.
Not every digression from the initial subject is a distraction. Some digressions have thematic significance; some digressions serve important roles like exposition, developing relationships, etc.; some digressions switch from one aspect of the plot to another one. But if you don't know why that bit of dialogue is there, if you don't feel that it adds to your story, than you do the same as with any bit of writing that isn't contributing: you change it, or you cut it out.
You will probably be able to tell the difference simply by assessing your own intentions while writing. If you feel that you're grasping for something, anything, for the characters to say, or that you're writing the dialogue based on loose association rather than sticking to some central point, then the dialogue is probably unnecessary. On the other hand, if you're trying to accomplish something (anything!) specific by changing topics, then you're probably fine - you can judge whether the topic change accomplishes your goal or not.
Studying real life conversations is fascinating, because they do ramble and wander, especially if the people know each other well. It is part of the richness that makes up real discussion, where tangents provide insight into the original discussion because they are tangents, and it indicates something about the mind working of the speaker.
I think there is a place for a novel where the theme is "love and loss", in which the characters do not talk about either of these topics directly, but the theme is developed by their skirting around them. It would not be easy to write, but would work by drawing the themes in relief - the particular topics that they do not talk about. And you would learn a lot because of the way that the conversation had to avoid them.
So yes, talk about other stuff, talk about life, because people do. And use this to show their thinking about the bigger issues. Where your core themes are big ones, then all issues will touch on them. A favorite breakfast was the last one I had with George. Or the one we took on our first holiday, in the open air.
