Steven who was speaking at a Q&A session at the Hay Festival in Wales told the audience:
"David owned that role in a spectacular way, gave it an all-new cheeky sexy performance and became a national treasure. And he didn't do it instantly, he did it over time. So the idea that Doctor Who could go on at all in the absence of David was a huge question, I didn't realise how many people thought it wouldn't succeed at all. That was quite terrifying when I found out about it later.
I think there were plans maybe to consider ending it. It was Russell saying, you are not allowed to end it."
Doctor Who will return with Peter Capaldi as the Doctor in August.