No film can tackle a true story about a hostage situation – in a bank, for good measure – without expecting to be compared with Sidney Lumet’s Dog Day Afternoon, a frazzling, profoundly compassionate high-water mark of 1970s Hollywood realism. 892, starring John Boyega as Brian Easley, a desperate ex-Marine who threatened to blow up a branch of Wells Fargo in the Atlanta suburbs five years ago, is an honourable entry in that tradition: limited, but honourable.

Boyega is at a point in his career where he deserves all the dramatic roles heading his way. This performance proves why. He digs down ardently into the part of a broke, tragic, PTSD-afflicted vet of Kuwait and Iraq, who had been scraping by on a monthly disability cheque from the Department of Veterans Affairs since his honourable discharge in 2005.

In 2017, that cheque, for $892, didn’t show up, and Easley’s rent was overdue. He called the VA crisis line, and they’d hang up. After a set-to with staff at their regional benefits office, he was humiliated and thrown out. His mental health was already in a parlous state, and his relationship with his eight-year-old daughter, from an estranged partner, unlikely to improve if he was having to sleep rough.

When he walked into the bank that morning, wearing a backpack and grey hoodie, he first requested a $25 withdrawal from one of the tellers – “big bills” is his quip, in Kwame Kwei-Armah’s script – before slipping a piece of paper over with the words “I have a bomb”. He had no bomb, just a fake detonator, but no one knew this for sure. His demands were few: for the bank staff to alert the authorities immediately, and simply arrange for the welfare arrears to be paid to him.

He let all the customers leave, keeping just the teller (Selenis Leyva) and a more senior bank manager (Nicole Beharie, typically excellent) on site as hostages. While they sat tight, police, including snipers, were deployed outside the bank. Frustrated and spiralling, Easley called a local news station, WSB-TV, and explained his predicament for over half an hour to the editor (Connie Britton) who picked up.

Every phone conversation in Dog Day Afternoon – between Al Pacino’s Sonny, the negotiators, and his lover Leon – seemed to prickle with intimacy and turn up the heat. It’s hard to watch 892 without feeling that it simply hasn’t captured the specifics of a situation nearly as well. The atmosphere doesn’t feel volatile so much as overdetermined. All the characters we care about – particularly Beharie’s – are sympathetically drawn into Easley’s plight, so the film is essentially pitting him against a system without a human face. While this might be an entirely valid take on the man’s experience, it’s less successful at getting the pulse racing.

In the hands of first-time feature director Abi Damaris Corbin, 892 splits the difference between Lumet’s film and John Q, the notoriously earnest 2002 thriller with Denzel Washington holding a whole hospital at gunpoint to give his son a heart transplant. At least dramatic tension isn’t ridiculously overcranked as it was in that, but instead, it sputters and stalls.

More than ever, Boyega has Denzel energy in abundance here, and it’s exciting to imagine how it will ignite in a scenario that feels a little less preachy. His phone calls with his young daughter are prime emoting opportunities, but the illusion that would build their relationship on screen is strangely missing – it feels like each is acting into a void. 

The film fares best when the chief negotiator, a fellow Marine vet played by the late, great Michael Kenneth Williams, steps into the fray. It’s one of his final performances, and a wary, angry one that elevates the material. His conversations with Easley touch a despairing nerve: this character himself is belittled, called “son” by a white colleague, and ultimately overruled in his efforts for peaceful resolution. In its dignified coda, the film rues another dog day, all round, for being black in America: a message that may be a little blunted in delivery, but still hits home.

Cert tbc, 102 min. Playing at the Sundance Film Festival. A UK release has yet to be announced