The director/actor plays police Inspector Kurt Wallander in the hit crime drama, adapted from the Swedish novelist Henning Mankell's books.
The upcoming third season follows Branagh's character as he investigates the deaths of young prostitutes, and the Brit admits he felt the need to talk to Mankell before shooting the difficult scenes.
He says, "I wanted to know how he was when he was writing this sort of stuff for months on end. He said he had to stop writing some books because it got too much. I asked if people thought he went too far sometimes - because the next episode is fairly extreme.
"But he said nothing he has written goes anywhere near the extremes of his research in terms of human trafficking, paedophilia and the appalling things people do...
"The darker the episodes get, the more people seem to like them - I don't know whether people look at them and think, 'My life will never get that bad.' When I read the script, I said to the producer, 'This is the bleakest thing I have ever read.'"
Branagh credits Judd Apatow's comedies with providing him with the light relief he needed during the emotionally draining shoot in Sweden last year (11).
He adds, "You end up using a bit of gallows humour to cope. I found I wanted to be around brightly coloured things and I watched a lot of very silly comedies - things that didn't involve me using my brain. I tuned into a lot of Judd Apatow comedies. This was my evening viewing - anything that would make me instantly laugh and make me be completely in the opposite direction."
Author Henning Mankell was among the activists aboard a Gaza-bound aid flotilla when it was stormed by forces intent on preventing safe passage.
Nine civilians were killed and several activists were detained by the Israelis, including the Swedish writer, according to news service AFP.
The activists were hoping to thwart a Gaza blockade on Monday (31May10) when several pro-Palestinians onboard clashed with commandos.
A spokesman for Sweden’s foreign ministry confirms Mankell is one of nine Swedes detained by the Israelis, who are facing deportation or criminal trials.