ONE of Hollywood's best-kept secrets is out. Chris Cooper, in his first major star billing, delivers the performance of his career in director Billy Ray's Breach, says OLIVER FENNELL.

In this tautly-directed espionage thriller, Cooper delivers a mesmerising performance as agent Robert Hanssen, a real-life FBI turncoat who spent the majority of his 25-year career selling American security secrets to the Soviet Union.

Breach tells the tale of how Hanssen was snared, thanks to a two-month operation which saw surveillance employee Eric O'Neill (Ryan Phillippe) go undercover to pose as Hanssen's personal assistant.

There is little action through the film's 110 minutes, and as we are told from the opening scene Hanssen will be arrested, there is only suspense in set-pieces.

But the story's appeal lies primarily in its characters. Cooper layers Hanssen with conflicting emotions, motivations, strengths and weaknesses. Not often are movie villains - even those based on real people - so complex.

Phillippe, too, gives strong support as the inexperienced O'Neill, whose career and marriage both buckle under the strain of his sink-or-swim assignment.

The narrative doesn't focus on the battle of wits as much as one might expect. In fact, it all seems too easy for O'Neill, whose strategy consists of little more than saying what Hanssen wants to hear at opportune times.

Another gripe is the lack of detail about the kinds of secrets Hanssen sold, or the implications of his actions to the FBI and, indeed, the country and its people.

It's frequently highlighted he was the most dangerous spy in American history, but, sadly, few illustrations why are given.

Perhaps, since Breach is based on real-life events, it is sensitive information, but this dulls any sense of danger surrounding Hanssen and any sense of triumph when he is finally apprehended.

But again, it is Cooper's performance which makes the film stand out. No pantomime villains here - his Hanssen is a man with a brilliant mind but a heavy burden, a devout Catholic with a sexual perversion, and an iron-fisted boss who craves recognition.

And Cooper has a fascinating face which can switch between warm, intimidating and pathetic with just the slightest of tweaks.

Cooper has been a reliable performer throughout his career (Adaptation, American Beauty), unanimously respected by film buffs but relatively unknown to casual viewers. Breach should change this - the secret is out.