Mississippi Grind, which screened at the Melbourne International Film Festival this month, opens with a brief shot of a rainbow emerging into view over Iowan farmland, bookended by seconds of black screen. The separation of this image from the bulk of the film that follows it makes a kind of thematic sense, since the rainbow itself becomes a totem for the film’s lead characters: two inveterate gamblers road-tripping down the Mississippi river toward a high-stakes poker game. The rainbow is a symbol of their felicitous bond, and the pot of gold they hope awaits them. But, of course, nobody ever made it to the end of a rainbow.

The gamblers are Gerry (recently resurgent Australian actor Ben Mendelsohn), a hopeless addict, and Curtis (Ryan Reynolds), a charismatic stranger who saunters into Gerry’s poker game and comes to seem like the personification of good luck. With Gerry deep in the hole to an array of increasingly impatient creditors, he begs Curtis to stake him on the trip south and accompany him as his lucky charm. Like its over-determined opening image, the film, by writer-directors Anna Boden and Ryan Fleck, is a touch heavy-handed but still charming, thanks in large part to the efforts of Mendelsohn.

Gerry is a complicated mixture of openness and inscrutability. Like a true long-term addict, he’s always ready to admit his flaws, because he knows it’s inevitable they’ll eventually become obvious. ‘I have problems with money,’ he whispers to one young woman, forestalling the possibility of romantic connection. ‘I’m not a good person,’ he tells Curtis, as their road trip loses its way.

When he says these things his eyes are steady, his hands spread open – a picture of honesty in extremis, since honesty offers the only dignity available to him. But his capacity for self-diagnosis is also a kind of resignation. Because he knows these things about himself, he knows he cannot change them. An intelligent man whose awareness of his own addictions cannot help him master them, Gerry offers the perfect vessel for Mendelsohn’s uniquely expressive talents as a performer.

Mendelsohn’s reinvented profile has been built on a series of turns as hard men and dirtbags. Much of his current fame can be traced back to his appearance in 2010’s Animal Kingdom as Pope, the volatile older brother of a Melbourne crime family. He was an odd fit in The Dark Knight Rises (in 2012), as a be-suited business rival to Bruce Wayne, but then at home again in Andrew Dominik’s Killing Them Softly (2012), as a logorrhoeic junkie, and in Starred Up (2013), as a violent career prisoner. This year he starred in the Netflix TV series Bloodline as the black sheep brother to a family of prosperous Floridians, a performance for which he’s received some of the most prominent American notices of his career.

Despite his ease with playing lowlives, there’s an uncommon delicacy to Mendelsohn’s style. Look again at his entrance in Animal Kingdom, where Pope, on the run from the police, tries to sneak in the back door of his Melbourne home, and surprises his nephew, J (James Frecheville), in the process of fetching some beers. Mendelsohn practically sidles into the kitchen. He almost seems shy of the scene; twisting his body away from the camera, and barely making eye contact with Frecheville. Without ever seeming deliberate about it, he takes over the frame.

There’s a similarly reticent body language at work in Grind. The film begins with Gerry sitting in his car, listening to an instructional CD on poker tells, before shambling into a casino. A security guard offers him a friendly ‘good luck’, and Mendelsohn answers with a nod that’s more like a slump. Once at the card table, he slouches down and away from the other gamblers, while never taking his eyes off the action. These opening moments offer one of the most effective bits of character business of any film this year. Almost nothing else need be said about Gerry’s self-loathing and self-resignation.

There’s something nearly flirtatious about Mendelsohn’s style as a performer. He always seems ready to dance away from his scene partner, and from the audience, but there’s a nakedness to his body language ­– the shifting of his eyes, the set of his mouth, the flutter of his hands – that invites attention. His expressions are filigree-light and quicksilver-fast, but he rarely offers up any obvious or rote psychological cues. Gerry’s face is a floodgate of emotional tells, but Mendelsohn stills keeps his card close to his chest.

He shares a palpable chemistry with Reynolds, and side-by-side the two offer a small study in contrasts. Reynolds has seemed on the cusp of stardom for over a decade, since he graduated from American sitcoms and buffed up into a leading man type. Although he’s a capable performer, his specific utility as an actor has yet to become apparent, and he’s been knocking around romcoms, comedies, and superhero movies for a while without ever finding a home. He has seemed perpetually at risk of being eclipsed by any number of fair, squared-jawed actors that have followed him down the Hollywood pipeline. Chris Pratt now has the career that once seemed Reynolds’ for the taking.

Reynolds is effortlessly charismatic, but his range sits squarely on the line between smirky glibness and a brown-eyed soulfulness he communicates largely by posing his eyebrows into a quizzical slant. Curtis is a fitting role for him, as he is mostly just required to strut around being attractive and putting his sitcom timing to good use. Fleck and Boden feint at a mysterious past for Curtis – there are intimations that a troubled history that has caused him to turn into a rootless wanderer – but there’s not a lot of depth to him. He’s ultimately a Peter Pan type, treating every new acquaintance as a potential lost boy to string along on his adventures.

Gerry, thanks to Mendelsohn’s technical facility, registers as the more particular character: a genuine personality, weighted down by a history of error, and all the more inscrutable for it. His authenticity suggests the possibility of a more difficult film, but Boden and Fleck are ultimately too enamoured of their characters to offer up anything overly challenging. Despite Mississippi Grind’s gritty milieu and rough edges, the filmmakers clearly want to leave Gerry and Curtis, and the audience, in a comfortable place. As a result, they spin the film a little too far on the roulette wheel of possible conclusions. But when Mendelsohn’s in play, the result still feels up to chance.