The Harry Potter star appears in upcoming BBC show Birdsong, an adaptation of a war novel by author Sebastian Faulks, and she shed her clothes for a scene with Redmayne.
Poesy grew close to the Brit during filming after they shared their own failed romance stories - but the actress still found her role opposite Redmayne daunting.
She tells Britain's Elle magazine, "I usually try to get rid of them (sex scenes) but they were so integral to the story here. You just worry about how to make them good.
"Eddie and I spent three days in a small room talking about our failed relationships. We had such little time and, because you shoot so quickly, you have to trust the person in front of you.
"Talking so openly was like stripping naked in front of each other and letting go of all the protections and inhibitions we had. The scenes are awkward, though. There's always that moment you think, 'When is someone going to shout 'cut'?"