The subtext – the thing that’s happening this time that you haven’t really quite seen with the Doctor before is… we’ve seen him fall in love loads of times, we’ve seen him have secret crushes, we’ve had him… we’ve had all of that very much in the Doctor – I don’t think we’ve quite seen …

It wasn’t so much the sadness, it’s that both the Doctor and Clara are people who think they know better. That’s what they are. The Doctor always thinks he knows better; Clara always thinks she knows better. So what they do at the end is a perfect summation of their relationship. They lie and endure …

I think from “Mummy on the Orient Express” forward, we’re getting a really interesting dynamic between Clara and the Doctor, where the companion is the most like the Doctor they’ve ever been, and whether or not that’s necessarily healthy. Two Type-A people in one TARDIS. https://www.nerdist.com/2014/12/steven-moffat-on-clara-becoming-the-doctor-in-series-8/