Septimus knows his own arrogance, though not necessarily the extent of it; he knows he will do what is necessary, though he doesn't like it; and sometimes he underestimates how cold he can get, and when he doesn't tries to actively avoid it. But his father's line all have certain potentials and despite Septimus' inability to follow through on some of it... it is there.

And sometimes, it shows.

With a crooked smile, Sep sighs as well. "I appreciate it. I've little enough use for it, here, and my friends used it. Not always, of course."

He looks down almost moodily. It's still hard , and it will take direct questions to keep him talking and not deflecting, but at least he's talking at all.