The Krays is less a biopic and more a character study of two sociopathic, narcissists. A difficult task for seasoned actors, I should have thought, but for relative acting newcomers, the Kemp brothers (Gary only had three previous acting credits, and Martin, only one) it must have been a mighty challenge. But, it's a task, to which, they are both equal.
Gary plays Ronnie Kray as a terrifying mixture of dark, brooding energy and ferocious, sadistic fireball. Martin plays Reggie, the 'sensible' twin, as a more measured, perhaps more 'normal' man, whose character flaws – a narcissism equal to his brother's and a vicious temper – put pay to his marriage (which ends with the suicide of his wife, Frances (Kate Hardie)), and the life of thief and thorn in the twins' side, Jack 'The Hat' McVitie (Tom Bell).
Gary and Martin are both brilliant as the East End crime bosses. The Spandau Ballet pop stars are every bit as good as actors as they are as music icons. Their performances are captivating. They both exude charismatic menace, and ably stand shoulder-to-shoulder, in acting terms, with legends Billy Whitelaw and Steven Berkoff.
The movie is not completely factually accurate, but I don't think that really matters. What does matter is how well the actors do with what they're given, and I don't think any of the principal cast have any worries on that score.