Peaky Blinders indeed comes with the most accurate casting, and what makes it accurate isn't its realism in a biopic sense but the connection of every cast member with their character conceptually. Every actor is born into the role's psychology. The casting seemed accurate, not because the actors looked similar to their historical counterparts in Peaky Blinders, but because they are aligned with their characters' physicality, voice, and thematic intent.
For example, Steven Knight did not just bring some gangster looks to the screen but also his capability to communicate trauma and emotional damage without dialogue. In Peaky Blinders, most of the characters are war survivors and therefore are emotionally fractured and morally compromised. Cillian Murphy, Paul Anderson, and all of them were able to pay justice to this emotionally complex persona.
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**Disclaimer: The article below is the author's opinion; the reader's discretion is advisable.
Here is how Peaky Blinders comes with the most accurate casting in the crime genre

As mentioned, the alignment between the actor's psychology and the character's backstory is what made the Peaky Blinders casting stand out among all the crime genres. Cillian Murphy is accurate to the idea of Tommy Shelby, who is a reluctant leader and a tactician. On the other hand, Murphy comes with a fragile body structure and can portray haunted stillness on his face.
His character could have been portrayed as a crude alpha male, but his ability to use silences, pauses, and his gaze made him brilliant and accurate for the right portrayal. Many fans on Reddit also discussed how, when they see Cillian Murphy bored on a random video outside Peaky Blinders, they could see Tommy in his eyes. He resembles a man who survives on calculation and good planning, not brutal force. His character reflected the situation of many post-war leaders who were damaged, intelligent, and withdrawn from life.
Similarly, Paul Anderson portrayed Arthur not just as a violent and unstable brother of Tommy but also provided a background history of what made him who he is today. Anderson gave him shame, religious guilt, and a childlike need for approval. This is what made Arthur's character even more complex, as he became someone who could be a brutal killer on one hand and could also cry after kneeling for regular prayers.
But I think casting Helen McCrory as Polly Gray is where the show quietly wins. This casting grounded the show not just politically but emotionally. McCrory doesn't perform the typical "strong woman" trope, but she plays authority through restraint.
Even actors who were playing supporting roles, like Sam Neill, Tom Hardy, and Adrien Brody, were all portrayed to have a different energy, but it doesn't seem to be a miscast. Like Hardy, Alfie Solomons felt unpredictable without being chaotic, and this is what made him a perfect cast for what the show intends to portray.
This is what made Peaky Blinders casting different than any other crime show

All of the actors were popular with recognizable faces, but none of them felt like they were breaking the film world. They submitted their psychology and craft to the tone, dialect, and rhythm of the show. Especially in long-running crime dramas, this continuity is rare to be seen.
The stories of crime always come with moral ambiguity and dilemmas, traumatic pasts, or life experiences that have transformed a civilized human into committing a crime, sometimes to take revenge, other times to prove themselves. Basically, someone who has survived something and has learned a wrong lesson out of it.
Therefore, I think Peaky Blinders achieved what most crime shows don't because it understood that casting isn't just about authenticity of accent but also about faces that can hold moral ambiguity just with the depth in their eyes, without dialogue or explanation.