Although he is highly regarded in Hong Kong as an actor (nominated for Hong Kong Film acting awards four times since 2007, winning Best Actor for Ching yan (2008)), Nick Cheung (Breaking News, Exiled) seems to me very inexpressive. As undercover policeman Harry Sin, Cheung is onscreen in every scene of “On the Edge” (I think the English title is more apt than the Cantonese one for the 2006 movie “Hak bak do,” which means “black and white route” for a movie about gray areas). I’m sure he does not smile. He looks anguished and depressed in every scene, both the present-day ones after he has engineered the arrest of charismatic triad (gang) leader “Don” (as in Mafia don) Dark (Francis Ng [The Mission, Infernal Affairs II, Exiled]) and in flashbacks to the four years in which he was Dark’s trusted lieutenant and was romancing a bar girl called “Cat” (Rain Li, who is also a cinematographer, coshooting “Paranoid Park” (in English) for Gus Van Sant.
There are quite a lot of movies from Hong Kong and elsewhere about the pressures of being an undercover park, most notably the “Infernal Affairs” trilogy that Martin Scorsese remade as “The Departed,” “Donnie Brasco,” etc. These generally show the policeman on the job, perceived by other policemen as well as by gang members as a gangster. With (often disorientingly unmarked) flashbacks, “On the Edge” is mostly about Harry Boy trying to function as a regular/open policeman after the big bust.
His former friends, not least his best one, Mini B (Derek Tsang), whom Dark set up in an ice cream parlor after Mini B lost the use of an arm in a fight with members of another gang, consider Harry a traitor to their benevolent leader (Dark). The policemen, not least the aged without promotion, violence-prone Officer Lung (Johnnie To regular Anthony Wong), who had roughed up Harry, not knowing he was a cop, distrust the de facto rookie with lots of gang associations and no friends on the force, having been put into undercover operations right out of the police academy.
Harry still has feelings for Cat, and she for him, though she is not going to marry and settle down: she enjoys her work as a bar girl. Nick has put out word to his followers not to take revenge on Harry. Nick has a measure of respect for Harry having outplayed him… and must be aware that Harry did not report some things to the police, not least the torture and killing of the man who incapacitated Mini B’s arm, but Harry cannot count on support or protection from either his fellow policemen or his former gang friends. He wants to be a good cop, and wanted that from the start of his long undercover assignment. In short, Harry has good reason to be on edge and depressed… so that what I think are Cheung’s limitations (affectlessness, unchanging dour look) fit with the part of the self-loathing character. (I can’t help noticing that Cheung is about ten years older than someone eight years out of the police academy, however.)
My favorite scene occurs early, as Dark has been surrounded by police cars with Harry having a gun to his head. Dark asks Harry how long he has been an undercover cop. “Eight years.” Then, “How long have you been working for me?” “Four years.” Then with mock relief that is at least partly genuine, Dark expressed relief that Harry did not sell him out (he says he’s glad that it was not underpaying Harry that led to his arrest) but was a crime fighter, however covert, before they met. (In a way, as the chases bookend the movie, the scene in which Lung talks to Harry in the end in some ways complements the one with Dark from the start.)
That exchange followed an elaborate car chase, and there is another one near the end, and some fight scenes in between, though most of the confrontations in the movie are not brawls.
It seems to me that Herman Wau (Untold Story) did not direct the interesting script he cowrote with the panache of Johnnie To. There are no bravura tracking sequences, nor are the images shot by Pucinni Yu (Taxi Hunter) distinguished.
I think the movie coulda and shoulda been better, but the supporting cast, especially Wong and Ng, lift it above the average gangster/triad genre movie.