Perhaps it is because he shadows Haiti's military ruler, Lieutenant-General Raoul Cedras, the man facing down President Bill Clinton and the international community. Or perhaps it is because of the more than shadowy nature of his work for General Cedras, in a PR role that makes 'dirty tricks' sound euphemistic.
Mr Garrison, who collects vintage airplanes and flew stunts in the film Those Magnificent Men in Their Flying Machines, is the man behind the recent smear campaign against Haiti's elected but exiled President, Father Jean-Bertrand Aristide. His description of the soft-spoken priest as a 'psychotic manic depressive with homicidal and necrophiliac tendencies', and his description of the President's alleged calls for 'necklacing' opponents (setting fire to petrol- filled tyres around their necks), formed the basis of CIA reports that caused splits in the United States over policy towards Haiti and may have led Mr Clinton to back away from his earlier commitment to Mr Aristide's return home.
There is little hard evidence to back up the allegations against Mr Aristide, whose unarmed supporters have turned violent only after massacres by para-military gunmen, and whose followers see him more as a Gandhi figure than the madman recently portrayed.
How can one Canadian adventurer, who speaks neither French nor Creole, whose own mental make-up is at the very least unusual, have played such an important role in a country whose people are dying of terror and famine? Because, it seems, the CIA took him seriously, and his interpretation of diaries, paintings and medicaments he 'liberated' from Mr Aristide's private headquarters after General Cedras' coup in September 1991.
Mr Garrison, who served as a fighter pilot in the Canadian Air Force from 1964 to 1971 before plying his skills in Hollywood, is, by his own account, in contact with the CIA and Republican senators, including Jesse Helms and Robert Dole. It was Mr Helms who branded Mr Aristide a 'psychopath' on the Senate floor recently. Mr Dole has expressed similar sentiments to discourage Mr Clinton from risking the lives of US troops on Mr Aristide's behalf, democratically elected or no.
Mr Garrison was called to Haiti by the coup leaders 'to lend a hand' the day before they overthrew the populist President in 1991. He came, he insists, merely as a 'friend of Haiti', unpaid (although provided with a bodyguard), and has been here since, often sleeping at military headquarters as a security measure.
His first task? To go through Mr Aristide's private possessions at the palace, according to the rare interviews he has given. He found Mr Aristide's diaries and handed copies over to a friend, Colonel Pat Collins, the then US military attache in Haiti who is now in Mogadishu. Mr Garrison kept the originals for his private collection at his Los Angeles home.
Mr Aristide's doodlings of eight-headed monsters, a common voodoo symbol, led to many of Mr Garrison's later allegations. Then there were the nave paintings found on Mr Aristide's walls, some showing people being tortured and killed by what is known here as 'Pere Lebrun'. Pere Lebrun was the name of a former tyre manufacturer in Haiti, whose advertisements showed a smiling black face sticking his head through a tyre. In Haiti, the 'Pere Lebrun' paintings, like those of voodoo ceremonies that depict the biting off of chickens' heads and people in trances, are common. But Mr Aristide's collection was apparently what led to the allegations that he advocated what in South Africa is commonly known as 'necklacing'. Contrary to reports circulated in the US, Mr Aristide never mentioned 'Pere Lebrun' in a speech before the 1991 coup, although he did refer to 'that wonderful smell' - which could have been an abstract reference to the practice. His actual words have been non-violent.
Mr Garrison also keeps the paintings in his Los Angeles collection, along with Mr Aristide's pyjama top, which he claims is of a voodoo design. The fact that his own military and police bosses practise voodoo as much, if not more, than the next man, Mr Garrison apparently considers irrelevant. Michel Francois, the Port-au- Prince police chief, recently visited a renowned hougan (a voodoo priest) called Dieupere and sacrificed a bull in an eerie night ceremony, according to witnesses. Colonel Francois was apparently invoking the help of voodoo spirits in his face-off with Mr Clinton.
Along with General Cedras, Colonel Francois was supposed to step down and allow Mr Aristide to return by 30 October under the UN-brokered Governor's Island agreement.
Mr Garrison has also made much of Mr Aristide's medicine cabinet, whose contents he keeps in a box at military HQ and claims back up the allegations over the President's mental health. But those who have seen them say the medicine bottles appear designed for a man with heart trouble rather than mental problems.
Nevertheless, Mr Garrison's one-man campaign may have tipped the balance in the US as Mr Clinton kept US marines on alert for a possible intervention. While ostensibly continuing to back the exiled President and his return, US officials began using phrases such as a 'weird, flaky guy' and Mr Clinton himself made a faux pas when, in an attempt to back Mr Aristide, he said: 'Look at the alternatives.'
Mr Garrison has also accused Canada's ambassador to Haiti of having irregular links to the Libyan leader, Colonel Muammar Gaddafi. One wonders if it is his influence at work when virtually every Haitian official who is attacking opponents raises some alleged connection to Col Gaddafi.
A US cameraman, waiting to film an interview with General Cedras last week at military headquarters, was idly twiddling a small automatic camera. 'He took your picture,' said a Haitian woman who appeared to have a close relationship with Mr Garrison. The cameraman had not. But the Canadian 'friend of Haiti' ripped out and exposed the roll, saying: 'A man could die for this.' At the cameraman's insistence, he replaced the film with a fresh roll.