There’s one thing Oscar Isaac does first whenever he receives a script he’s remotely interested in: he sends it straight to acting and voice coach Gerry Grennell.

“It’s usually like, ‘I think this is interesting, I wonder what Gerry thinks’.”

As Isaac explains to The Hollywood Reporter, Grennell will then usually send back detailed notes to the actor, considering areas such as what he would be searching for in the role, how he might interact with the work as both a performer and a human being, and if it offers something new for him to explore. “And by this stage, he knows me so well,” says Isaac.

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But the star isn’t the only one to seek out this unassuming, charming, hilarious and exceptionally chatty Irishman’s thoughts before embarking on a new project. 

Over almost 30 years, the Dubliner has quietly amassed a client list that reads like a who’s who of Hollywood’s very best, including multiple Academy Award winners and some of the biggest names working today. Several have made Grennell their default go-to whenever they’re prepping for a role (although he humbly suggests he uses “sleight of hand” to convince them of his value).

Alongside Isaac — who he’s teamed with for more than a decade, encompassing his star-making turn in 2013’s Inside Llewellyn Davis and his steep rise since to one of the most in-demand leading men across both mega-budget studio tentpoles and prestige art-house films — Grennell has worked with the likes of Marlon Brando, Meryl Streep, Natalie Portman, Sean Penn, Tom Cruise, Josh Brolin, Cillian Murphy, Bruce Willis, Anne Hathaway, Matt Damon, Brian Cox, Ewan McGregor and Orlando Bloom. Johnny Depp and Andrew Garfield have both been frequent collaborators (“If you are lucky enough to be taught by Gerry you should listen,” Garfield once said), as was the late Heath Ledger. 

It was Grennell who Ledger contacted when he was offered what would be his defining, and Oscar-winning, role in The Dark Knight, helping him examine and develop his performance as the Joker in the Batman film. More recently, Grennell worked with Pedro Pascal ahead of The Last of Us, and prepped Boyd Holbrook — another regular (he’s described Grennell as an “absolute legend”) — for the upcoming Indiana Jones and the Dial of Destiny. Fellow countryman and newly minted Oscar nominee Paul Mescal is among the newer clients, the two chatting over a number of sessions for his upcoming film Carmen. 

There’s good reason Isaac jokes that he’s “apprehensive” about adding any further fuel to Grennell’s profile. 

“He’s my special wizard, like a Rumpelstiltskin who makes all of my art for me,” Isaac says. “And nobody can know!” 

While he may be regularly introduced as an “acting coach” and his long list of credits are predominantly either “dialogue coach” or, more succinctly, “coach,” Grennell himself admits these labels are misnomers.

“I’m not really an acting coach. I’m a facilitator, perhaps, or a maybe you could call me a conversationalist,” he says, speaking to THR from his home in the Irish capital, where he also gives regular intensive workshops to budding young stars at the Bow Street Academy (a school he helped establish by devising its — and Ireland’s — first ever one-year screen acting course). “But films don’t have those categories, and productions have to have a name. And honestly, I couldn’t give a shit what you call me” 

But sometimes he is simply there to help actors perfect a particular accent, which is where it all began.

Having set up an experimental theater company in his youth with some friends in Dublin, Grennell says he soon realized that while acting “was something I could talk about,” it wasn’t where his skills lay. Although the stage may not have beckoned, he did possess a growing fascination with the art and science of the performance — “about what happens to your state of mind when you’re in the performance situation” — something that had developed while studying music in Germany (he played the guitar), where part of the focus was on performance psychology.

This would eventually draw him towards teaching, as would being diagnosed with Repetitive Strain Injury — which impacted his vocal abilities — earlier in his life. He was introduced to the Alexander Technique — the movement training method adopted by numerous actors, and those suffering from RSI, to, among many things, reduce tensions and improve postural habits — while at his theater company. Realizing there was a lot more to learn, he headed to London, first for four years on an Alexander Technique teacher training course and then to the Central School of Speech and Drama. 

It was there where his teacher Julia Wilson-Dixon — herself a noted dialect coach (she worked with Robert de Niro on Mary Shelley’s Frankenstein, Glenn Close on Albert Nobbs and Eddie Redmayne on The Theory of Everything) — spotted Grennell’s talent with accents, inviting him to work on a job she couldn’t do. 

Grennell’s first film was the 1994 drama A Man of No Importance, starring the “delightful” Albert Finney and following a poetry-loving bus conductor in 1960s Dublin who wants to stage Oscar Wilde’s Salome with his passengers. Principally hired to help Finney and co-stars Rufus Sewell, Tara Fitzgerald and Michael Gambon polish their Irish brogues (although he admits, being Irish, Gambon’s was already solid), he wound up “working with everybody.”

More gigs would soon follow, but it wasn’t long before he was expanding beyond merely fine-tuning accents (as he did with Streep on 1998’s Dancing at Lughnasa, helping her achieve the soft northern lilt of County Donegal). His obsession over performance — not just how to make a sound, but how to use it — began bubbling to the surface of the conversations he was having with the actors.

“By definition, it’s going to come up, because once you open your mouth, then it’s performance,” he says, explaining that the rhythm of particular accents, alongside intonation, are hugely influential on how they are delivered. 

“Intonations are driven by the emotions rather than the intellect, and then, if my emotional palette is the equivalent of a painter’s palette with colors, then I need to know what I’m doing with this stuff.”

The first real move into fully-fledged performance coaching came with Ledger, who he would meet via a somewhat ridiculous connect-the-dots map of stars, one that began in the small Irish town of Ballycotton in the summer of 1995. 

While filming on the drama Divine Rapture may have famously ground to a halt after just two weeks when it emerged that the production company’s escrow account was non-existent, the ill-fated feature proved to be a major turning point for Grennell. 

Not only did he get to work with Marlon Brando (although he insists that “you don’t work with Marlon”) and used every opportunity to “interrogate his genius,” it was on set where he developed a close friendship with his co-star Depp, with the two bonding over music between scenes. 

In what would become a regular occurrence with each new creative talent drawn into Grennell’s orbit, Depp — then one of the hottest stars in the indie film world (and in general thanks to his then-relationship with Kate Moss) — decided to take his new acquaintance along for the ride, the two working together on on several more movies (including, much later, Alice in Wonderland, Grennell helping whittle the 11 accents Depp had originally wanted for his Mad Hatter down to three, one with a hint of Irish). 

An earlier project for the pair was 2001’s Jack the Ripper gothic horror From Hell, shooting in Prague, where he met Ledger, there filming one of his first lead roles in A Knight’s Tale.

“He came to set and we got chatting, and then I went to his set and got chatting, and it was really nice — we got on,” says Grennell, who was soon being invited to join Ledger as the young Australian’s career erupted, from The Four Feathers to The Brothers Grimm, Brokeback Mountain, Casanova and The Dark Knight. “It became a very hand and glove kind of relationship — he’d send me scripts and we’d talk about them, and whenever I wasn’t working on something else, he’d ask me to work with him.”

Grennell was on a shoot in Texas when he got the call from his friend saying that he’d been cast as the Joker, and moved with his family up to Brooklyn to be close to the star as they began examining the script and character (he later lived with Ledger in London during the shoot).

“At first, it was a very casual, relaxed, shooting-the-breeze thing,” Grennell recalls. “But by definition if the topic on your mind is The Dark Knight and your specific role, no matter what you talk about it’s going to be flavored by elements of what might be curious to you about it. So my job there would be to be a listener, and also a provocateur, to stick a couple of pins in this area and see if they get a reaction.”

Pins — plus a great deal of research into personality traits, studying case studies and reading and re-reading the script, examining all the characters to help “construct your perspective of who you are” — would eventually culminate in Ledger’s unnatural yet iconic voice for the Joker. 

Grennell was working closely with the actor on what would be his final feature, Terry Gilliam’s The Imaginarium of Doctor Parnassus, the two once again sharing a flat as it filmed the London. Grennell later explained to the press how he would see Ledger battling with the crippling insomnia that would eventually lead to him tragically and accidentally overdosing on prescription medication in 2008 at the age of just 28, something he describes a “desperate loss.”

Like Ledger, Isaac was an actor who was quick to identify Grennell as both a friend and uniquely skilled collaborator when they first met in 2010 on the set of Madonna’s box office flop W.E. Grennell had been coaching co-star Abbie Cornish, and Isaac says he found in him not simply a kindred spirit prone to “compulsive misbehavior,” but an individual who was “completely disarming and truly authentic.” 

They soon began working together, Isaac noting that Grennell transformed his relationship with the Alexander Technique — which he’d already been taught in acting school — into something much deeper and more spiritual.

“He talks about relating to the world and relating to space as a performer — how you relate to the material, how you relate to character and all these things have these roots in the Alexander Technique and even deeper roots into the nature of consciousness,” he says. 

Grennell’s impact on his work, Isaac claims, was immediate, and as his career took off he became a constant creative presence by his side, on email and over the phone. “It’s rare that there’s a project I do without him.”

For Denis Villeneuve’s sci-fi epic Dune, arguably his biggest role to date, Grennell provided Isaac with the methods to train his voice to stay in the “chest register,” as he wanted for Duke Leto Atreides. But he also delved into the character, concluding that Leto was a leader very much aware of his own fate on the desert planet of Arrakis and someone who required “pragmatism of gargantuan proportions” as he sought to achieve his goals before the inevitable. It was this conversation that Isaac says led him to speak to Villeneuve, and, in a scene with Rebecca Ferguson’s Lady Jessica, give the Duke the resigned line, “I thought we would have more time.”

In a more recent credit and one that has become one of the standouts of 2023, Grennell helped prepare Pedro Pascal for HBO’s highly acclaimed video game adaptation The Last of Us. Due to the pandemic, he wasn’t able to visit the series’ set in Canada, as he would usually do, instead speaking to the actor online before the shoot (as he did with Mescal, with Carmen having shot in Australia). 

Part of the conversations he had with Pascal was around the believability of on-screen relationships between characters. His job, he says, was then to “analyze how you create an environment where those relationships are organic,” and ensure audiences don’t subliminally mistrust any moments of intimacy they’re seeing. An element to the answer, he claims, lies in “your relationship with yourself.” 

Again, Grennell’s credit on the show as “coach” may not quiet cover the full breadth of his activity, remote or not. 

With comical levels of self-deprecation, Grennell is repeatedly at pains to downplay whatever it is he may bring to an actor’s skill set or repertoire, despite the ever-growing list of major projects and names on his resume (not that it’s one he ever updates — a friend adds credits to his IMDb page as he hears them, and while he does have a LinkedIn account, Grennell says he “can’t remember the password.”)

“Honesty, I’m just full of shit. I have to be unguarded about what I say and there’ll be a whole lot of bullshit,” he claims of his conversations. “But every now and again there might just be something of value. It might switch on a light in an area that would be of interest.” 

The actors that he works with, he says, are extraordinary, akin to thoroughbred racehorses, with or without his interference. 

“They’re wired up in a particular way where nature has allowed them to navigate the world of expression that is open, free and without prejudice,” he says. “In my sort of innocence, I just sit there in awe and soak it all up. And every now and again, I get the chance to bask in their sunshine.”

But for Isaac and a legion of stars now banging on his door, Grennell has been an instrumental figure in giving them the creative tools to dial up that sunshine, through methods that keep them coming back for more (with further A-list referrals coming in thick and fast).

“All he does is open up avenues of expression in a really fun way,” says Isaac. “He’s never been prescriptive. For some people, that’s what they want, they want you be like ‘tell me what shape my mouth is supposed to make when I make that sound, and I’ll take care of the rest.’ But the joy with Gerry is that it’s like a toy box of fun things.”