They must have thought about the risk, especially when the book was, unexpectedly, a huge success, and the film was not only optioned (lots of books do, and nothing ever happens), but then made, with two big stars.
I imagine you could sort of get sucked in to the process, though. A delighted agent and editor telling you sales are huge, publisher publicity person booking you in for festivals and tv appearances, the film option getting picked up, the producer casting two stars etc etc — you’d sort of keep forging ahead, knowing it was all likely to come crashing down, preparing what you are going to say…
ETA. Which does partly explain why they both came across slightly oddly in interviews. I saw that one other people had referenced, Rick Stein talking about their cider-making enterprise, and thought ‘Oh, these people are a bit strange.’