Doctor Who is many things, but apparently it's not the kind of thing you can stick on and then not watch when a 'special friend' comes over.

At least that's according to Rachel Talalay, who directed the two part series finale this year. On her Tumblr blog, she notes that the Byzantine Heaven Sent is not your usual Saturday night fare. "The episode requires attention," she notes, "I don’t think it’s 'Netflix and chill'."

Indeed, 'chilling' is the last thing on their mind when producing an episode. It's well worth reading all of Talalay's notes on filming, which give some impression of the sheer effort involved. For instance, just these two lines:

Triggers all of these considerations:

2nd unit/stock for sky? 

We ended up taking a stock sky shot and animated the moving flare, rather than the more common moving clouds shot. (Note that the flare is moving in the wrong direction.) 

For the light sweeping in the room – find a room where we have access to windows where we can put lights on cranes and create the movement. Smoke will enhance this? 

We found these logistics in the Great Hall on the main floor of Caerphilly Castle, where the lighting could be controlled for the moving beams and they fortunately did allow us to use movie smoke. Lights were on lifts to provide enough height for the shadows to hit the floor to emphasize the movement. 

This could have been done with VFX as well. 

While at another point she ponders:

Which corridor?

Blood? — very not-Doctor-Who.

At this point, I  don’t have any idea who this is. Later I know and realize nothing can be given away.

My corridor of choice was at Cardiff Castle, but we were not permitted to use blood at this location. Must re-think.

Very, very not 'chill'.