Tom Hiddleston, British rising star (as he has been officially been anointed by BAFTA in this year's Awards shortlist), pauses to think of something.
"The pressure I put on myself to get it right," he comes up with eventually. "I first saw Raiders of the Lost Ark when I was eight, so I wanted to be ready for anything he threw at me. I wanted him to be able to ask, 'Tom, I want you to lead a charge of 120 horses...' I didn't want to be the weak link, a problem person, but because everybody is feeling like that, there probably are no problem people."
Hiddleston, despite being an actor much in demand on both sides of the Atlantic, is clearly still reeling from being taken under his childhood idol's wing, particularly when he describes their first meeting.
"I'll never forget it - time slowed down. I got a call from my agent saying Steven wanted to meet me. I thought it had to be a joke. The whole thing felt like a movie, driving to see him past all the studios with the tours and everyone on the buses. Then there's this little hut where his office is… and I remember saying to myself, 'Just be yourself.'
"I met his assistant first, she made me a coffee, bizarrely we got talking about Guinness. Suddenly he walked in, saying 'I love Guinness!' So, there we were talking about Guinness, then the First World War, my family and stuff. Quite suddenly after about an hour, he said, 'I'd like you to do it.' I was stunned - I had to ask him to repeat it."
It's been a big year for this sweetly-mannered 30-year-old. Straight off the back of a role in Thor, he’s been Rachel Weisz's leading man in The Deep Blue Sea, Spielberg's straight-backed officer, and even F Scott Fitzgerald for Woody Allen in his well-received rom-com Midnight in Paris. Some of these parts have been bigger than others, so what means the most?
"They say there are no small parts, only small actors," he reflects. "Every part I play, I throw the kitchen sink at it. You're trying to give truth to a human life that isn't your own so, however small it is, you still have to work very hard to create such authenticity.
"I'm only in Midnight in Paris for ten minutes, but I'm still playing F Scott Fitzgerald, so I have to get it right, and reach for him."
If authenticity is Hiddleston’s key goal, is he troubled by the chasm between what he went through as an actor on War Horse - high-octane horse-rides and falls into mud punctuated by a cup of tea and a massage in the trailer, possibly - and what actual soldiers at the Somme experienced?
"I don't feel self-conscious about it, although I wouldn't trade places with anyone," he muses, rubbing chin. "I often imagine who I might have been in these situations - a hero, a coward, an alcoholic - and the answer is I never know. But I do believe in the power of stories to move people, which is the reason I became an actor. Films have always broadened my understanding of the world, so if War Horse does that as well, I've done my job."
Hiddleston, a classics graduate, is deeply steeped in the history of war, and happily refers to the classical Greek versions of what we call 'quest' and 'honour'. Where does he think these can be found in what is, thankfully, a peaceful existence for most compared with the horrors of the past?
"The closest I've got to it is when I used to play rugby, and there are no bayonets or rifles, or horses, but you're putting your body on the line for something. It sounds so trite to compare rugby with warfare, but I think the warlike spirit of man has been channelled into sport in the modern age.
"We still need to compete and it's much better to do it on a sports pitch than it is to actually try to kill each other. I suppose they thought they were doing it for king and country back then, and you can do it for a team, something to defend, a pride, an identity.
"That goes back to the odyssey, the search for the ultimate prize, glory, the choice Achilles had to make - go home now, live a long life, have many children, die old and happy, or stay and die, but his name will live forever.
“So Achilles stays and dies and here we are talking about him.”
No wonder Steven Spielberg liked talking to him and gave him a job.
War Horse is in UK cinemas from today.