When it came to actual filming, though, one of the biggest challenges for Riley was the early sequence in which he, with a sword, goes up against Capaldi's mighty spoon.
"The fight with a spoon was actually incredibly difficult," he says. "Because it's tiny. It's a spoon! So my aim had to be bang on, and Peter's blocking had to be bang on. It's not like a sword where you've got a foot length to clash into – you have to be so precise, but make it look like you're struggling. We spent ages on that rehearsal."
"Also," he adds, "Capaldi was adamant that he'd fall in the water, so he got properly wet suited up and I was like, 'Am I going in too..?' So right at the end of the day, freezing cold river, Peter's got this suit on, and then it was decided, 'You know what, actually? Tom, you should go in too.' But I didn't have a wet suit on... That laughing is very forced at the end! You can't see how cold it is..."
From that fight onwards, the Doctor and Robin Hood develop into one of the funniest double acts ever seen in Doctor Who, and the Doctor even starts to like him a little towards the end. Would Riley ever be up for a return to Sherwood forest?
"I'd love to! I had an absolutely brilliant time on it. It's the great thing about the show. Because it constantly reinvents itself every week, there's a whole new cast, whole new setting, whole new time period – all the creative people on it never get bored. . I would completely go back." Radio Times
MTV: Tom, Robin Hood is such a completely different character from Da Vinci on “Da Vinci’s Demons.” How did you get cast in the role, and what was it like creating this take on Robin Hood?
Tom Riley: It was great fun to do. When the offer came through, it was one of those things that rather than, should I do this, it was, I have to do this. We made sure the scheduled was free, and the gap was going to work. On the page, it came across as absolutely hilarious anyway, but once I talked to Mark over a cup of tea, he talked about how he saw Errol Flynn in the movie, and I had seen it a long time ago but went back and refreshed myself on it. I was absolutely fascinated by how out and out shamelessly heroic Errol Flynn played it, and unapologetic, and full of joy. As far as The Doctor was concerned, the Peter Doctor, it was going to rub him the wrong way, and that was an absolute blast to play.
Gatiss: What’s very nice, and what emerged as we did it is the idea of how much front he has, that he’s doing for a purpose, that he’s being a hero because that’s what people need. And underneath it he has a nice backstory. He’s lost Marian, he needs to prove himself. But Clara sees through him instantly, I love that scene in the camp where she says, “The Doctor is right, you’re laughing too much.” It is for a reason. He’s not just a cardboard hero, he knows why he has to behave like this for the people around him.
Riley: It’s that stuff in the script that lets you lean to the more outlandish side of his personality, because I was aware that he was more character than caricature. That gives you freedom, liberty to play knowing that he’s rooted in something completely real, as well as seeming impossible.
MTV: I wanted to ask about the Robin Hood laugh, actually, because it’s amazing. How’d that get developed?
Gatiss: When I watched the film several times in preparation, it struck me straight away. They laugh at the drop of a hat! Literally. They laugh at the drop of a hat. I just thought it was a really funny idea that the Doctor would go, “Stop doing that! Are you simple or something?” So then it becomes a sort of motif, and the Doctor finds it becomes infectious, The Doctor is bantering and he hates it. But Tom developed that laugh, and its excellent.
MTV: So did you try any other possibilities, Tom? I don’t know how we’ll get this across in a text piece.
Riley: [Laughs] Oh man. I tried the Errol Flynn, and I saw that very deep, hearty, inclusive laugh, and thought, well, that’s what it is, isn’t it? It’s infectious, and as long as everyone around plays in, as the Merry Men did, all laughed as much as Robin, then it’ll work. I was going to go for a thin, reedy, creepy one, but it just didn’t play. [Laughs]