I can't imagine how I never came across this before, but I've discovered that The Paris Review did one of their "how-I-do-it" interviews with Mr. Stoppard. This is from back in the Winter 1988 as Hapgood was about to be staged. That's about six years after The Real Thing.
From the interview:
As I was doing it I watched a documentary about Crick and Watson’s discovery of the structure of DNA—the double helix. There was only one way all the information they had could fit but they couldn’t figure out what it was. I felt the same. So the answer to your question is that the rehearsals are going well and enjoyably, but that I’m very busy with my pencil.Here's a direct link to the .pdf
