This film marked the beginning of the Alastair Sim 'Green' period which lasted ten years – from 'Green for Danger' to 'The Green Man'. During this decade Sim became one of the best-loved (and unlikeliest) of British Film stars of the post-war years. His lugubrious face, which had the eyes of a recently kicked bloodhound, and distinctive voice with its clipped diction and hooting vowels, lent themselves equally to comic and sinister characters, and occasionally combined both in the same role ('Hue and Cry' and 'Scrooge'). After years as a respected character actor, 'Green for Danger' was Sim's first truly leading role, and he was an instant hit as the detective/narrator tracking down a killer in a wartime hospital.
In a sense the film is one long flashback as Sim's Inspector Cockrill dictates his report on events at Heron's Park Hospital in August, 1944. Suspense is immediate as we learn that two out of six people in an operating team will be dead in five days' time.
The south of England is under attack from the V-1 flying bomb, and in one incident the local postman, Joseph Higgins, (Moore Marriott, unrecognisable from his role as old codger Harbottle in Will Hay films) is slightly injured, needing an operation to set a broken leg. During the operation the following morning Higgins' breathing becomes erratic. Despite the best efforts of the team he collapses completely and dies. Someone has switched a black and white oxygen cylinder for a repainted green (for danger) carbon dioxide cylinder.
The film reveals more background to the main characters, with particular emphasis on a love triangle/quadrangle between Dr Barnes (Trevor Howard), surgeon Mr Eden (Leo Genn), Nurse 'Freddi' Linley (Sally Gray), and Eden's rejected earlier fling, Sister Bates (Judy Campbell). At a staff party that evening Sister Bates reveals Higgins' death was murder: she knows how it was done, and the culprit. Going to the operating theatre to retrieve the evidence, Bates is attacked by a figure dressed in an operating outfit and stabbed to death. Inspector Cockrill arrives the next day.
Cockrill is rather eccentric. He has a pronounced fear of the V-1's. He loves his work, and doesn't object to press interest (“they always give me a good write-up”). He also enjoys the reactions he provokes in others, whether it's the morbid curiosity of onlookers, indignation at a slyly sarcastic insult, or outright violence between two jealous men (“an LRCP mixing it with a leading surgeon” he says gleefully, sitting back and grinning with pleasure). He also has an aptitude for quoting poetry.
Normally we would expect these 'eccentricities' to hide a first-class detective brain which leads Cockrill unerringly to the killer. In fact, the Inspector is as flawed as any of us. He is not all-knowing (Sim's reaction when he fails to deduce the outcome of a detective novel is a highlight in the film), and just as likely to be led astray by his assumptions. In several ways, 'Green for Danger' is both a classic whodunnit and a sly spoof, as everything the audience thinks it knows turns out to be wrong, and we have to start rebuilding our knowledge, and re-evaluate our own deductions, in the light of new information.
The film as a whole works thanks to its strong characters and the performances of the actors playing them. Although based on a novel where character can be developed at greater length and in more detail, the film does not skimp on their development and relationships. The tension between the characters rises, and the mystery and suspense are sustained until the final minutes, where things are still not as they seem. The audience's primal fears of hospitals, as places of sickness and death, where sinister masked figures operate on a body over which you have no control, are also played to the hilt.
The atmosphere of the hospital (filmed entirely at Pinewood studio) is very well presented, with its dark, shadowy corridors and an air of makeshift wartime improvisation, very expressively photographed by Wilkie Cooper. The music of William Alwyn also emphasises mood, switching from the high-spirited title theme to swooning, sinister 'murder music' in the operating scenes.
Ultimately, however, 'Green for Danger' belongs to Sim, and it is a great pity there was not a series of Cockrill films to follow this one.