American culture has become so globally dominant that even the lamest U.S. customs, such as our soporific presidential debates, infect countries blessed with superior traditions. For example, as part of the run-up to the May 6th General Election, the Brits are holding their first ever prime ministerial debates, although their party leaders come up through the gauntlet of Parliament’s vastly more substantive and scintillating daily Question Time.

That the prime minister has been evolving into the president junior is a recurrent theme of Robert Harris’ 2007 novel The Ghost, which Roman Polanski has made into The Ghost Writer. This competent political thriller for grown-ups, one of the better movies of 2010, has been playing in limited release in the U.S. and opened over the weekend in Britain.

The PM’s job was long more human-scale job than the president’s, less insulated from normal life by security and by deference (which in Britain was paid instead to the Queen as head of state). For example, when I attended a conference with ex-PM Margaret Thatcher in 1999, she showed up accompanied only by a secretary and a bodyguard, wearing an old dress that had been mended with needle and thread. Tony Blair and his money-hungry wife Cheri were the first to indulge fully American superstaritis.

Ewan McGregor (Trainspotting) portrays a self-effacing hack hired to render readable the stultifying memoirs of a Blair-like retired prime minister played by Pierce Brosnan (the former 007). This handsome and rather blank politician, whose decision to help America invade Iraq has left him more popular on this side of the Atlantic, is holed up behind suffocating security at the wintry Martha’s Vineyard estate of a Rupert Murdoch-style media billionaire who has given him a $10 million advance.

The previous ghost writer somehow fell off the ferryboat, shortly before the ex-PM’s arch rival within the Labour Party ratted him out to the International Criminal Court for handing four English-born Muslim fanatics in Pakistan over to the CIA for “€œextraordinary rendition.”€ A war crimes conviction would strand the politician permanently in exile in America, the only upscale country not to recognize the ICC.

Polanski’s involvement raises obvious ironies. The director of 1974’s Chinatown was himself on the lam from the law for a third of a century after drugging and anally raping a 13-year-old girl in Jack Nicholson’s house. (The rapist is currently under house arrest in a Swiss chalet.) Because Polanski would have been arrested if he filmed in America, a dreary German resort island stands in unpersuasively for Martha’s Vineyard.

Not surprisingly, Polanski, who lost his mother to Adolf Hitler and his wife to Charles Manson, can”€™t seem to get too horrified by the idea of a British PM taking strong action against Islamist crazies. He lets Brosnan’s character have the final word on how to make airliners secure from terrorists.

Yet, all that’s beside the point because, despite the auteur theory, The Ghost Writer isn”€™t really the director’s movie. It’s the author’s.

The plot and dialogue are lifted as faithfully from Robert Harris”€™ novel as John Huston’s movie of The Maltese Falcon was straight out of Dashiell Hammett’s book. (Legend holds that Huston’s secretary simply typed up the novel in screenplay format for the first draft.)

Polanski was originally going to make a big budget version of Harris”€™ historical novel Pompeii, but switched to the English author’s more topical (and cheaper) next book. Harris writes intelligent, well-researched thrillers, such as Fatherland and Enigma, in the mode of Frederick Forsyth (The Day of the Jackal), an earlier BBC reporter turned first-rate middlebrow novelist.

While Forsyth is a Conservative Euroskeptic, Harris is a New Labourite whose English nationalism looks with alarm more west than east. A friend and advocate of Blair turned bitter enemy, Harris seems less enthused by the International Criminal Court than he is incensed by Blair selling out British national interests to the White House. Although he”€™d likely strenuously deny it, there’s a streak in Harris of the late Enoch Powell, the brilliant classicist and reactionary Tory patriot who relentlessly deplored the U.K.’s foreign policy being reduced to that of a satellite of the U.S.

Polanski becoming a satellite of Harris is a little incongruous considering the filmmaker’s greater artistic renown. Yet, middlebrow novels, with their reliance upon plot and dialogue rather than descriptive prose style, have always served as more trustworthy sources for movies than literary works.

The main problem with Polanski’s straightforward adaptation is that, like most stories about writers, The Ghost Writer works better as a novel than a movie. The film is good, the book is better.

Unfortunately, writers are visually boring. Harris”€™ book is full of fascinating information about the craft of ghostwriting and why the memoirs of politicians, despite being furnished with the best writers money can buy, usually turn out so dire. But all that has to be rushed past in the movie.

(The one advantage of film over text for this story is that with a book, you can tell which one is the ultimate plot twist just by how many pages are left.)

Blair’s autobiography, by the way, is due in September. His publisher insists he employed no ghost, which may explain its unpromising title—Tony Blair: The Journey.