In December 1968, the cabinet of Brazil’s ruling military dictatorship gathered for a classified meeting which resulted in the issuance of Institutional Act No 5, a decree that stripped dissenting citizens of their civil rights and led to a blood-soaked period of forced disappearances, torture and extra-judicial killings. While the meeting was recorded, the tapes only emerged in recent years. João Pedro Bim’s documentary overlays contemporaneous propaganda newsreels with these damning recordings to create an intriguing juxtaposition that reveals the covert machinations of dictatorial rule.

Roused from their archival slumber, the newsreel images conjure a mirage of prosperity and unity. In these state-produced materials, marching soldiers, modernist new-builds and flag-waving patriots are ubiquitous. The recorded statements made by high-ranking officials at the infamous meeting accompany these signposts of social harmony, as the plan to restrict democratic freedoms is laid out clinically and methodically. Cast in a new light, the smiling faces that populate the jingoistic films turn eerily grotesque.

If propaganda functions as a kind of intellectual opium, Behind Closed Doors provides a rigorous detox through its contrasting of official narratives and concealed abuses of power. Bim accentuates this distancing effect on a visual level by adding freeze frames, time lapse and jump cuts to the existing footage. Such manipulation of continuity pokes holes in the facade of collectivity projected by the state.

Though fascinating in their way, these stylistic devices become repetitive. As the film progresses, the various pairings of sounds and images offer fewer and fewer surprises. Moreover, considering how similar some of the past authoritarian tactics are to the strategies enforced by Jair Bolsonaro’s recent government, it is perhaps a missed opportunity that the film didn’t make a more explicit link to current politics.

• Behind Closed Doors is on True Story from 21 March.