Which brings us to the $64,000 Question: how do you do it?
Obviously, there's more room to play with Cass in the fanfic than in the books -- if you can't hold a viable, paying audience in fanfic, there's no problem. If you can't do that in the real world, people lose their jobs. Even in fanfic, though, you need to watch what you're doing in order to make sure the character remains recognizably Cassandra Cain. Which isn't easy; DC can't always do it in the source material. And they're really, really trying. DC keeps trying different stuff, to see what works and what doesn't, and I think they miss a fair portion of the time.
This is what I think works: brevity and simplicity.
(Nota bene: my take, not DC's. DC has done any number of things; this is my view on what *works.* I've written Cass twice: once in "New Year's Kiss," my first Batverse story, and once in the opening of "Jason and Me." You can judge for yourself what you think of her voice in those stories, and how well I pulled it off.)
I think Cass's dialogue works best when it's kept short. Her vocabulary isn't bad, and she has a pretty good grasp of grammar, considering. What she's not good at is putting it all together. When she speaks, she uses only a few words at a time. My rule of thumb is that three-word sentences are about the max of her comfort zone. More than that takes effort, and she'll either pause mid-sentence or wait before speaking so she can put the sentence together, rehearse it in her head, and make sure it sounds right.
At the same time, Cass is *fluent* in those two or three words. And she can put several mini-sentences together for a longer block of speech. But if she can say something in fewer words, she'll do it. Because (and this is something I think DC should bring back) from Cass's point of view, speech is inefficient. When you're used to knowing what somebody's thinking and feeling with a glance, then that person telling you what they think and feel is tedious by comparison. I think Cass's attitude toward words might be sort of like her attitude towards having a secret identity -- maybe useful for some things, in theory, but not much of the stuff Cass wants to do. In other words:
Speech is just so much blah-blah-blah. Cass is about precise control. No wasted movements, no wasted words. That's why this feels so off to me -- I'd have taken it the other way, and have Bruce drop into near-Cass-speak. B: "Namali?" C:"Lying." B:"The translator -- a spy?" C:"Yes. Not for him." B:"The Sakuri Lions." C:"Namali knows." B. "Hmm. [pause] And the people?" C:"...At war." Stuff like the bit in #51, where Cass uses slang, because she's watching TV to try to speak like other people... not so much. Partly because it's just painfully unfunny.
(An alternative take is that Cass could be a very, very good listener, maybe giving occasional nudges to get people talking in a direction that would get their body language reflecting what she wants to know.)