Sometimes when writing fiction I just need a line of dialog to move the story forward. I want to throw it out there and move on. It doesn't need to be smart, I think. It just needs to get the job done. But that feels unsatisfying.
There are solutions. Sometimes you can skip over the cliche bits by summarizing. Sometimes you can just start the scene after the cliche bits. And then there's a third option which I wouldn't usually consider: You can expand on it.
This passage from Guards! Guards! (by Terry Pratchett) illustrates that third approach nicely:
"Hwhat," [Lady Ramkin] said, "is the meaning of this?"
If a Ramkin had ever been given to introspection, she would have admitted this wasn't a very original line. But it was handy. It did the job. The reason that cliches become cliches is that they are the hammers and screwdrivers in the toolbox of communication.
This line of dialog – being so straightforward and familiar – could have been a throwaway to move the scene along, but Pratchett lingers on it. The paragraph suggests Lady Ramkin is not introspective – "If a Ramkin had ever been given to introspection." And then he goes for a digression on cliches, which itself invokes a cliche of the "toolbox of communication."
(Somehow I understood this excerpt paragraph was funny before – while writing – I noticed what was funny about the paragraph: The cliche in the justification of cliches. That's an interesting thought.)
Thinking about this, this seems like one of Terry Pratchett's common tools. Exchange or line of dialog, then a paragraph explaining or expanding on it in a way that both develops a character and makes a joke. I've definitely seen this before in the eight other Discworld books I've read. It's frequent. Not every line gets this treatment, of course, plenty of passages are pure dialog. But at least a few times per book.
Why expand on this particular line? Not sure. Best guess, it's a combination of: (1) it would otherwise be a boring line, (2) she's a main character, (3) it's the beginning of the scene, good to spice things up a little at the beginning. (1) and (2) seem like the primary things here.