Benedict Cumberbatch, the British actor whose high-profile portrayal of WikiLeaks founder Julian Assange is generating Oscar buzz, has launched a defence of the man he plays in new movie The Fifth Estate. "I think we show his ideas and integrity and self-sacrifice," said Cumberbatch, the morning after the film opened the 38th Toronto film festival. "I think there's a lot to celebrate about his achievements."

In the film, Cumberbatch plays Assange as a quicksilver saviour, humane at times, deceitful at others, never less than human. The actor, 37, said empathy was key to his interpretation. "I think to try and go into this realm of thumbs up or thumbs down is so limiting. You want to find what's human about him. And that's not to soften the edges. [But] so it's something we can relate to."

This qualified warmth was echoed by the film's director, Bill Condon, whose back catalogue includes biopics of similarly contentious figures such as sexologist Kinsey and the film director James Whale. "Watching the movie," said Condon, "is the experience of being impressed and turned off by Assange every five minutes."

"He's an absolute pioneer," Condon continued, "and he's made a huge difference. He opened the door that Edward Snowden just walked through. For all of those reasons I think he's an extremely admirable figure." Condon also defended the decision to include a lot of biography about the Australian hacker in the film — including anecdotes about his background and flashbacks to an unhappy childhood — by arguing it was impossible to separate Assange's public and private selves. "There isn't a disconnect. It's all part of the same puzzle."

The Fifth Estate is based partly on the book by Guardian journalists David Leigh and Luke Harding about Assange's rise to international notoriety, and in particular, his collaboration with this media organisation, among others, in publishing thousands of classified diplomatic cables and war reports. The investigative reporter Nick Davies, who first reached out to Assange, is played by David Thewlis in the film, while Peter Capaldi plays editor-in-chief Alan Rusbridger. Dan Stevens, who plays former deputy editor Ian Katz, joined Cumberbatch and Condon at the press conference.

Stevens, famous both sides of the Atlantic for his role in Downton Abbey, recalled how he'd met the former Guardian deputy editor for lunch just before catching the train to Brussels for the shoot. "It's fascinating just getting behind what it was like being a journalist at that time," he said. "The danger and paranoia these guys were feeling. For some of them, for the first time they were fulfilling what they became journalists for." The experience of playing a newspaperman, said Stevens, had left him with an enhanced sympathy for "good journalists" and heightened criticism of the bad.

Cumberbatch refrained from engaging with questions about how he thought the impasse between Assange — currently living inside the Ecuadorian embassy in London — and the US government eager to extradite him would end. "It's very complicated and I'm not a legal activist. What I'd like to see is the man able to carry on able to do his work as the founder of WikiLeaks. Beyond that, due process has to take place in whatever shape of form."

The actor has three major films playing at the festival. As well as The Fifth Estate, he's part of the supporting cast of Steve McQueen's hotly tipped 12 Years a Slave, and the adaptation of Tracy Letts's Pulitzer-prize winning play, August: Osage County. Such visibility had led to Toronto's artistic director, Cameron Bailey, naming him man of the festival. Cumberbatch responded to the tag bashfully, as he also did to questions about his own tech prowess (there are multiple scenes of complex coding). "I interface with easy, user-friendly software on sleek-looking hardware. [This] was a education, but not one I took to naturally."

Assange himself had asserted early on that he wanted no part in the film, which also finds a source in the account of working with him by former colleague Daniel Domscheit-Berg. He had seen an early draft of the script and dismissed scenes — subsequently omitted — which were set in Iran. Cumberbatch ventured that he would likely still feel the same, despite the nuanced portrayal. "I'm not a betting man but I imagine he won't particularly want to support the film. But we'll have to wait and see."

• First look review: The Fifth Estate