Though his documentary “Touching the Void” earned him a BAFTA, Kevin Macdonald has segued into narrative films like “The Last King of Scotland” — which won an Oscar for Forest Whitaker — and the new thriller “Black Sea,” starring Jude Law as a submarine commander.

What made you think of Jude Law as a rough-and-tumble Scottish submarine captain?
Jude has always been great when he’s done character roles. In this film, he really committed himself in a big way. He aged himself, he put on a lot of weight and he mastered this difficult Scottish accent.

For someone who didn’t intend to make fiction films, you have a way with actors.
Mostly I’m just in awe of them. I could never do what they do. When an actor gives a great performance, it’s the closest you can get to magic, because you don’t fully understand how they made
that person come so alive.

I heard shooting underwater was particularly tough.
We put foam in the water to make it soggy when the actors treaded on the bottom; it had a fire retardant that chemically reacted with the chlorine in the pool and turned everything bright green. I came in one morning to find the water the color of Limeade.

What did you do?
We had to drain all the water into the outdoor tank at Pinewood Studios, which was being used as the Nile for Ridley Scott’s “Exodus.” So he was turning that water red at the same time we were pumping green into it.