Joaquin Phoenix said he was surprised to discover a version of Napoleon who was more like a soppy "teenager in love" than an all-conquering commander as he researched his epic new role.

Ridley Scott's "Napoleon", which hits cinemas worldwide next week, features massive-scale battles across Europe. 

But it is also a portrait of Napoleon's complex relationship with his wife Josephine, played by Vanessa Kirby, which has been preserved in the general's often tragically pleading letters. 

"He was very socially awkward. I think of him as a romantic with a mathematician's brain," Phoenix told AFP in Paris. 

"He wanted to be heartfelt but in his letters… he seems like a teenager in love, almost plagiarising poetry.

"There's something almost endearing about it — if he wasn't also responsible for the deaths of millions of people," Phoenix added.

"I imagined that he was cold and calculated as a great military strategist. What I was surprised by was the sense of humor and how childlike he was."

Phoenix, 49, said he had waited more than 20 years to work with Scott again after their huge success with "Gladiator" in which he played another emperor, Commodus. 

But the director didn't call until "he had a story about a petit, petulant tyrant, and he said 'I've got just the guy!'" Phoenix joked. 

The "Joker" star refused to be drawn into any cheap comparisons between the war-mongering emperor he plays and the conflicts currently ravaging the world. 

"If I was in the midst of a conflict, the last thing I'd want is to hear from some actor sitting in the Bristol Hotel," he told AFP. 

"There's such real pain and heartache people are experiencing right now and I don't want to conflate a movie I'm in, that cost a bunch of f—ing money, with something that's happening. I feel that's just wrong."

Kirby said the relationship between Napoleon and Josephine was fascinating but "exhausting".  

"I always found it amazing that this man who built an empire could write these letters," she said. 

"They were so inexorably drawn to each other but to me it never seemed sane, calm, healthy — it was obsession and infatuation and power dynamics that would swing," Kirby added.

The actors' research was complicated by the vastly different accounts that have come down through the centuries. 

"It's very hard to get a clear answer about many things," said Phoenix, who said his interest was in finding "inspiration more than information", through details like how Napoleon ate and drank.

"Some of it is ridiculous — two weeks before we were shooting, someone said, 'You know Napoleon was left-handed.' And then it took a week to disprove that," Phoenix added with a laugh. 

The same for Josephine. 

"Every book was completely different," said Kirby. "It made me feel she was an adapter… playing different parts to survive."