Benedict Cumberbatch and Martin Freeman in a scene from 'Sherlock.' The two also appear together in 'The Hobbit: The Desolation of Smaug.' BC/Everett CollectionBBC/Everett Collection

In the new movie "The Hobbit: The Desolation of Smaug," Martin Freeman plays Bilbo Baggins, a furry-footed "burglar" who must go up against a fire-breathing dragon named Smaug, played by Benedict Cumberbatch.

In the British television series "Sherlock," Mr. Freeman plays Dr. John Watson, sidekick and loyal friend to Sherlock Holmes —also played by Mr. Cumberbatch.

Messrs. Freeman and Cumberbatch performed separately using green screens in "The Hobbit" and barely saw each other on set, overlapping by "about three minutes," Mr. Freeman said. By contrast, they share many scenes together in "Sherlock," and they have completed filming of season three, which will air in the U.S. and U.K. in January.

Did they get up to old "Hobbit" hijinks or reminisce about Middle Earth while on the set of "Sherlock"?

Martin Freeman appears as Bilbo Baggins in The Hobbit: The Desolation of Smaug Warner Bros. Entertainment Inc. and Metro-Goldwyn-Mayer Pictures Inc.

"No," Mr. Freeman said with a laugh. "Obviously we acknowledged it, and we would talk about it, but 'Sherlock' is so intense. There's time for mucking around, but the mucking around is more 'Sherlock'-centric by then."

"The Hobbit: The Desolation of Smaug," to be released Dec. 13, is the second in Peter Jackson's "Hobbit" trilogy based on J.R.R. Tolkien's 1937 book. The second installment follows Bilbo and 13 dwarves as they continue their quest to the Lonely Mountain, the dwarves' former, treasure-filled home that was captured by Smaug.

Mr. Freeman is currently filming a new FX series, "Fargo," loosely based on the Coen Brothers movie. He spoke with the Journal about the next season of "Sherlock," beating up spiders and overcoming "tiny" acting. Below are edited excerpts from the interview.

In Mirkwood Forest, Bilbo has to defend the dwarves and battle giant spiders. We begin to see a ruthless side to him.

I was always impatient, where I would say to Peter, "When does Bilbo start to find that side of him?" He would always say, "It's coming, it's coming." That's the portion where he gives full rein to an aggression. He gets some of it from the ring, absolutely, but I think he's never been in the position before where he has to save people he cares about. It's literally life or death for him.

When he loses the ring briefly, there's a glimpse of Gollum in him.

It's certainly more fun to play than a gentle, slightly credulous, innocent Bilbo. That stuff is fun to play some of the time but it's also more fun to beat the s— out of a spider.

Were you literally wrapped in a spider web?

We really were wrapped, and it was very time-consuming to get in. It's kind of 90% imagination and 10% just a couple of stunt guys coming at you in a papier mâché green-spider thing.

There are many close-ups of your face but also action scenes in which you're sword-fighting and running from Smaug. Your performance is an equal blend of face and body.

For me it's a holistic thing. Even though I'm not your standard action hero, everything is a physical performance. I never really see the differentiation between what your head is doing or your face or your legs. What I do as Bilbo is absolutely the whole thing. It is a very different physicality from my own.

How is that physicality different in this film?

"The Hobbit" is a big movie, and the camera comes in on you. There is a heightened element to this. I have a problem telegraphing things, but I realize there is a sort of need and purpose in telegraphing something in this kind of film. If I'm there doing my tiny, tiny, tiny acting in the middle of this huge film, it's just going to get lost, quite frankly. You have to kind of get that vehicle across. I'm a big believer that acting is reacting, and that's a huge part of Bilbo. How he reacts to things is how the audience does, because the audience isn't a goblin or a troll or an elf. It's probably a hobbit, because hobbits are close to us.

In the scene where Bilbo and the dwarves are barreling down the river to Long Lake, did you think you could drown?

We were all in this huge set of 6-feet-deep water that was going very fast due to engines pushing the water into rocks. It was pretty hairy. It was the one sequence of the film where I definitely said a quick prayer every morning. You needed to concentrate, put it that way.

Did anyone get injured?

Yes, they did. I didn't, but there were a couple of injuries, and one of them was quite bad—crushed legs. My stuntman, actually, who is really, really experienced. Swims like a fish. He got his leg quite mashed, and as he said, and as I thought, "Thank God it wasn't me." That would have delayed production quite a long way.

When you filmed green scenes opposite Smaug, did you work off Benedict Cumberbatch's voice?

I was played a demo of what Ben did, so I knew roughly what Smaug was going to sound like. Then usually I would work into Leith, our dialect coach, and she was doing a very good Smaug. It was not Ben's voice, or a dragon, just a lot of imagining really, but reacting still.

On "Sherlock," what is your character Watson getting into in Season 3?

As far as he's concerned, Sherlock's been dead for two years, and so his life has moved on. He has a lovely woman in his life, and we join him probably a little bit sadder, but in another way content and happy because he's fallen in love. The last thing he's expecting is seeing his old friend again [laughs]. They gave us a really good script, and [the episodes] will be out in January. Writing that good with fantastic characters on a show that people love and are anticipating—you don't get many of those in a lifetime.

Write to Barbara Chai at