Some say that the face is the window to the soul. If that's the case, then L.A. Noire is going to have hoards of people gawping into its spiritual essence - fogging up the glass with their slack-jawed admiration. This is an enticing project from any number of angles - it's a 1940s-set detective game, and a Rockstar-published, open-world one to boot - but make no mistake, it's the faces that are going to set people talking.

The phenomenal phizogs / exemplary expressions themselves are the product of something called MotionScan, a performance capture procedure that works in a similar way to the tech used for Quantic Dream's Heavy Rain. Without delving too deeply into chin-stroking detail, MotionScan allows for the complete recording of an actor's facial performance - every twitch, snarl and grimace. But where Heavy Rain's digital cast existed within a relatively controlled environment, Noire's virtual actors operate within the potential chaos of a sandbox world - a highly detailed recreation of post-WW2 Los Angeles.

It's swiftly apparent that Noire will be something of a departure from Rockstar's previous open world titles. For starters, Rockstar is only the publisher here; the developer is Team Bondi, a Sydney-based studio formed in 2005. Then there's the protagonist, Cole Phelps - played by Mad Men's Aaron Stanton: he's a war veteran, a gifted detective, and appears to actually be quite nice. Sure, John Marston and Niko Bellic had their kinder moments - but Phelps appears to be a thoroughly decent chap. Unfortunately, the same can't be said for all the people around him; while Hollywood is booming, post-war LA sits atop a swelling undercurrent of violence and corruption. The game finds Phelps starting out on the Patrol desk and then sequentially moving through Traffic, Homicide, Vice and Arson. Within each of these departments you'll work through a set of cases, each of which takes the form of an extended quest. And while there will certainly be plenty of the action we've come to expect from open world games - shootouts, melee brawls and car chases - the meat of the game will take a more thoughtful, slow-burn approach.

You see, it turns out that L.A. Noire's faces are more than just graphical icing on the open world cake; they're an integral part of the gameplay. Phelps is a detective, and much of your play time will be spent conducting proper detective work. You'll arrive at a scene, search the area for clues, and perhaps consult with the local coroner. Then, crucially, you'll move on to interview witnesses and any other suspects who may have come to light as a result of the evidence you've unearthed. But here's the thing: where most adventures would use these conversations for plot exposition, as a bridge to the next helping of action, here the interrogation is the game. You pick apart your subject's statements, suss out their motives, cut through their lies. It's a verbal fencing match. And the best part of all this? You can actually see when someone is lying to you; their darting eyes and nervous ticks tell the whole story.

The conversation system in L.A. Noire seems to have parallels with the setup we'll see next year in Square Enix's Deus Ex: Human Revolution. When you reach a branching point in the chatter - usually just after a suspect has made a distinctly dodgy claim - you'll be given a choice of three reactions, each linked to a separate face button. Believe/Coax makes Phelps react in a gentle fashion, trusting the interview subject and encouraging them to say more; Doubt/Force takes a much harsher approach, pressing the interviewee to come clean and to abandon any lies or pretences they might be making. Finally, we have Disbelieve/Accuse, the harshest verbal approach in Phelps' arsenal. Here he'll directly confront the speaker over some kind of perceived infraction - whether it be something they've done, a lie they've told, or a piece of information they're still holding back.

When Phelps opts to Disbelieve/Accuse, the suspect or witness will invariably react with outrage - genuine or otherwise. To force an answer, Phelps must produce evidence to back up his claim. Every clue you find - whether it's a physical object or a piece of information - will be entered into Phelps' notepad, a vital tool that helps you to keep track of the various plot threads and personalities that arise during each case. Each fresh discovery will be pencilled down in neat handwriting, and every new character you meet will have their portrait sketched down as a visual reference. You won't just be limited to looking at details in the current case, either: as the story progresses, you'll uncover links back to crimes that you resolved earlier in the game. You'll have to be careful about putting too much stock in any one clue, however, as there are plenty of red herrings to stumble across.

Rockstar's first showing of the game focused on a case called The Fallen Idol, Phelps' final assignment on the Traffic desk. I'm slightly torn here, as I'm loathe to reveal too many details about such a story-driven game. As a compromise, I'm going to sketch out the rough outline of the quest while glossing over most of the twists and turns; with any luck this will give a solid indication of how the game plays (well, looks - this wasn't a hands-on demo) without giving away too many of the good bits.

The Fallen Idol begins with Phelps receiving a briefing at Police Headquarters: a car has driven over the edge of an embankment, almost directly across the street from the cop-shop. The vehicle's descent was prematurely halted by a billboard - a lucky occurrence that probably saved the lives of the car's two occupants: film star June Ballard, and a teenage girl named Jessica Hamilton. Phelps heads to the scene with his partner for the case, Stefan Bekowsky. Once he gets there, he swiftly discovers signs of foul play: the vehicle had clearly been rigged by an unknown third party, and Ms Ballard claims that she and Hamiton were drugged.

Right from the initial chat with the bruised actress, it's clear that things are going to get messy. During questioning, Ballard looks furtive and keeps biting her lip in a way that suggests she's perhaps not as good an actress as she thinks she is; confronting her with evidence from the crash, Phelps and Bekowsky extract hints that the perpetrator may be one of Ballard's Hollywood associates. The pair then head to the city hospital to question the younger of the two victims, and at this point the plot starts to take on a much darker tone, with the mystery leading deeper into the seedier side of the LA film scene. I won't spoil things by saying too much more, but as the story unravels the case takes us to a variety of seedy locations, dripping with noir imagery. One particular highlight finds our detectives visiting a gloomy prop house, guarded by a giant stuffed bear and several other dusty toys.

Geographically-speaking, Team Bondi's world isn't a 100 per cent recreation of '40s Los Angeles, but it's pretty damned close - and if you've been to the city you'll instantly recognise different parts of town. The world is packed with detail too, from the slick suits and art deco architecture right down to the traffic lights, with their little stop/go signs protruding on pivoted arms. Many of these little touches feed directly into the gameplay: when driving from location to location you'll hear radio chatter that may relate to your current case, or indeed to optional events that effectively act as sidequests; you might be on your way to interview someone when you hear about a minor crime occurring in your area - and if you so wish, you can then head off to deal with that before getting on with your primary business.

Neatly, the game seems to try its hardest to make life easy for the player without succumbing to bluntly obvious gameplay aids. When you've entered an area with clues, for example, the background score takes on a tingly, mysterious quality; when you've found everything there is to see, the music fades away again. Occasionally you may need to use the resources of the LAPD to help you with your investigations, stopping by one of the blue police phones that litter the side of the road; you might want to call in to check someone's background, for example, or to follow up on a partial license plate match. And whoever Phelps is partnered with for a case (it'll vary from job to job), they'll be on hand to actively help, rather than just bumming along for the ride. If you can't be bothered to drive to the next location, they'll take the wheel for you. And when you're about to interrogate someone, your buddy may offer subtle advice on how you should approach them. When Phelps first chats to young Jessica Hamilton, Bekowsky suggests that you go on easy on her, since she's just a kid.

It's the interrogations themselves that ultimately impress the most, however. It took me a good 15 minutes or so to get over the Uncanny Valley effect of the faces, so mesmerising is their impact. Few games have managed to convey expressions in such a convincing manner, and it's no small understatement to say that L.A. Noire could mark the start of a whole new vein of gaming. Uncharted 2 and Heavy Rain made great strides in the progression of storytelling and simulated emotion, but it's the way Team Bondi uses this verisimilitude that interests me. There's something entertaining and very satisfying about being able to spot when someone is lying to you, and while Ms Ballard's facial gymnastics were perhaps a little OTT, characters I saw later in the demo seemed far better at hiding their true feelings.

This bodes well, as surely much of the game's fun will lie in attempting to outsmart the NPCs. On the other hand, it sounds as if Team Bondi has designed the game in a way that allows for player error. In past detective games (Phoenix Wright, Hotel Dusk and the like) the tendency has been to force the player to re-try conversations until they hit the correct path, but apparently the cases in Noire will bend and stretch to accommodate various paths: if you fail to mine a nugget of info from one suspect, and alert them in the process, you'll have to find your answers from somewhere else. It's a promising setup, but a curious one too - because if a player is very astute at handling their witnesses, surely they may end up missing the content that will be shown to more clumsy players. It'll be fascinating to see how Team Bondi handles this design challenge.

There's also the question of action. This preview has focused almost exclusively on the investigation side of the game, but there's also the more explosive side of things to think about. At the moment, melee combat looks a bit less graceful than the rest of the game - it's still being tinkered with, apparently - although I did like the fact that Phelps' hat got knocked off as he rumbled; the car chase and gunplay sequences seem far more self-assured, and don't look a million miles away from their GTA IV counterparts, which is clearly no bad thing.

The bigger question, perhaps, is to what extent the game will use these mechanics. The Fallen Idol is the last case on the Traffic Desk, so it's only natural that we'd get a bit of action, but will there be a similar degree of action on every case? I'm so taken with the quieter, more thoughtful side of things, I almost feel it would be a shame to have too many bursts of action. Either way, I'm hugely excited by what I've seen of L.A. Noire so far. There looks to be genuine innovation here, both in terms of design and in the supporting technology. And with such a wealth of great source material to delve into - think Raymond Chandler, Dashiell Hammett and James Ellroy - Team Bondi's debut may be one of the major events of 2011.

L.A. Noire will be released in the Spring of 2011, on PS3 and Xbox 360.