Cleve Jones can cite the exact moment when Sean Penn morphed into Harvey Milk.
It occurred during filming of a crucial scene in Gus Van Sant's Oscar-nominated biopic Milk, which stars Penn as the former San Francisco city supervisor, one of America's first openly gay elected officials.
After honing his political skills as a flamboyantly courageous, bullhorn-toting community organiser, the so-called Mayor of Castro St decided to run for office.
He shed his ageing-hippie couture, cut off his ponytail and took to wearing conservative suits, the better to reassure anxious Pacific Heights matrons that he was a serious candidate.
When Penn emerged on set one day in that incarnation, ready for filming, Jones was struck by the actor's uncanny resemblance to his beloved friend and mentor.
"That was the day it all came together and Sean, like, had this direct line to Harvey," says Jones, a longtime San Francisco Bay Area gay-rights and labour activist who served as a consultant on the movie and appears in three cameos, in addition to being portrayed in Milk by actor Emile Hirsch.
"It was weird," Jones continues.
"It was eerie and wonderful and at times just incredibly sad."
A number of factors harmonically converged in the making of Milk.
In Van Sant, the movie found a director capable of imparting a resolutely independent vision to a film that's intended to play as well on Main St as on Haight St (San Francisco's famed hippie headquarters).
In Dustin Lance Black, it procured a screenwriter able to humanise and dramatise an opera-size chapter of American social history.
And with such supporting actors as James Franco, Diego Luna and the Oscar-nominated Josh Brolin as Milk's political rival and eventual assassin, Dan White, along with Danny Elfman's inventive musical score, the film-makers were able to conjure not only one man's remarkable story but also the turbulent sensibility of a mind-blowing epoch.
Still, none of these contributions would have added up without a lead actor who could bring focus and verisimilitude to Milk's ebullient, prismatic personality, an actor who could embody, rather than merely impersonate, the actual man.
Enter Sean Penn.
"I don't think anything could've prepared us for what he brought to the screen," says Bruce Cohen, who produced the movie with Dan Jinks.
"What we've heard from so many people is, you forget you're watching Sean Penn," Jinks concurs.
When White's bullets struck down Milk, and Mayor George Moscone, on November 27, 1978, America lost not only one of its most intellectually nimble and socially progressive politicians but also a man whom Jones calls "one of the most empathetic people I've ever met".
Among those who knew him best and worked with him most closely, Milk was cherished not only for his relentlessly determined leadership but for his wit, sensitivity and grace under pressure.
No-one associated with the film doubted Penn's ability to meet the role's enormous technical demands - indeed he is among the film's Oscar nominees.
Besides his professional credentials, he brought other experiences as a political activist and occasional globe-hopping journalist that made him seem a natural fit to play Milk.
Van Sant says that during the casting process, he watched a number of YouTube videos showing Penn giving speeches at town-hall meetings and in other forums, demonstrating his oratorical flair and charisma.
"He was funny, he was also daring and also accurate and extreme, which are a lot of things Harvey was," Van Sant says.
"We were very inspired by his talks."
As part of his research process, Black interviewed more than 40 people who had known Milk in various capacities.
His screenplay provided Penn and the other actors with the foundations of authenticity on which to construct their portrayals.
"He and I talked about the ideas somewhat," Van Sant says of Penn, who wasn't available for an interview, "but I think he kind of assimilated his character through studying Harvey and using his imagination and willing it into being."
Even so, certain aspects of Penn's screen image and popular reputation might have appeared to be at odds with Milk's persona.
"Maybe the surprise that Sean brings to it is because of his roster of characters being pretty macho," Van Sant says.
"That's kind of an interesting turn."
Jones acknowledges that, before meeting Penn, he "had this overall general impression" that "he was a blunt, possibly arrogant kind of a guy".
Then, during an early phase of the film-making process, he and Penn were discussing how the actor intended to portray Milk.
"He said, 'I'm just going to play him as a kind man'," Jones recalls.
"I felt tremendously relieved at that point."
During filming, by all accounts, the on-set atmosphere was highly amicable and relaxed.
"There were no meltdowns, there were no raised voices, there were no egos on parade," Jones says.
The cast cultivated a true ensemble atmosphere befitting the co-operative spirit of Milk and his political entourage during the volatile late 1970s.
Yet, for some, it was at times emotionally draining to relive those electric, traumatic years.
"The first couple of weeks, I was a blubbering idiot," Jones says.
Among the most difficult scenes to film was Milk's killing.
Jones, then 22, was one of the first people to discover Milk's dead body.
Watching that sequence "just about destroys me every time", he says.
But if parts of Harvey Milk's life and times were painful to re-enact, the film-makers share a sense of optimism that the movie has spurred renewed interest both in Milk and in the origins of the gay-rights movement.
Jones says that in recent years whenever he gave talks to students very few of them knew anything about Harvey Milk.
Now, he says, "I ask, 'How many of you have seen the film?' and everybody's hand goes up."
