He told Digital Spy:
“It’s a bit unbelievable to be bringing [the Zygons] back again – and also Osgood as well, another fan favourite.
I tried to do the biggest cliffhanger I possibly could.”
He also spoke about what it was like writing for two firm fan favourites:
“It’s a privilege to be telling a story about those two, and I think it’s quite an exciting story as well. Unfortunately I’ve got two fan favourites in my hands now, which can only set me up to disappoint people!”[source]