Benicio Del Toro is well cast as Che Guevara in Steven Soderberg's ambitious biopic of the iconic revolutionary. But how will a challenging, two-part Spanish-speaking epic about a long-dead Marxist play today? As a producer on the film, the Oscar-winning actor is keen to find out, he tells Donald Clarke

YOU HAVE to feel some sympathy for Benicio Del Toro. The charismatic actor has been saddled with the task of promoting one of the Oscar season's hardest sells.

Don't get me wrong. The Argentine, the first part of Steven Soderbergh's study of Che Guevara, is a very fine piece of work. Focusing almost exclusively on the iconic guerrilla's contribution to Fidel Castro's victory in Cuba, this sober film manages to drag the viewer along despite its unwillingness to sentimentalise or simplify.

But no templates exist for flogging a challenging four-hour drama released in two discrete parts. Del Toro not only stars as Che, but he is also a producer on the film. What's their strategy, I wonder.

"Nobody tells me anything," he bellows with a wide smile. "I mean, have you seen the stupid poster out there in the hall?"

I had, but I was too polite to mention it. The initial design for the poster (a much more attractive one has since appeared on billboards) features a hilariously stilted image of Del Toro - his fist in the air - staring grimly at the chortling viewer. He looks, for all the world, like Robert Linsday in the 1970s sitcom Citizen Smith.

"That's my head stuck on somebody else's body," he says. "The hat is different too. And he's doing the black power salute. They didn't do that then. He looks like GI Joe. I got to do something about that."

That's probably a good idea. But Del Toro will still have to break the news to the public that Soderbergh's film is to emerge in two parts: the first, The Argentine, is released on January 2nd; the second, Guerrilla, arrives the following month. Are audiences ready for such an eccentric approach?

"What makes me think it will work?" he says. "I don't know if it will work. We released the first picture in Spain, and I'm happy to say people did go and see it. You know, it is beyond my control if people will go and see it. But I like movies that continue to live. Maybe this is a film that will find a big audience in 10 years. Hey, it's a long game. It's a long movie!"

I like this Benicio Del Toro. In films such as The Usual Suspects, Trafficand Fear and Loathing in Las Vegas,he has variously seemed sinister, sombre and sedated, but, in the flesh, he turns out to be extremely good fun. Dressed from head to toe in black, he is an enthusiastic gesticulator and a big, big talker. Ask him, as I did, about the political history of Puerto Rico, his homeland, and you will be treated to a fiercely animated 10-minute lecture.

Was Che a hero to Puerto Rican kids when he was growing up? "No, no. I don't think so. I was not into him when I was a kid. I first read about him when I was 20 or 21. I read his letters to his family. So, I was impressed by him as a human being, as a man of flesh and blood. He was not a superman."

Del Toro is now 41. Surely T-shirts featuring that famous image of Che in his beret would still have been around when he was growing up? "The T-shirts came back in the mid-1990s. In the 1980s nobody was wearing that stuff. Before that, in the 1970s, The Clash had worn the shirt. But my generation fell into an abyss without Che. My generation was about Tom Cruise in Risky Business. I still wear the Ray-Bans, man."

This surprises me. In my naivety, I assumed that the citizens of Puerto Rico - just two islands along from Cuba, after all - would have had strong feeling about Che Guevara.

"Not then. Nada, amigo! I went to a good school to ninth grade and we heard almost nothing about Cuba. We heard about it when some Cuban sports team came, but what we heard was always bad."

Del Toro grew up in a middle-class corner of San Juan as the son of two lawyers. His mother died when he was just nine and, four years later, dad moved the brood to Pennsylvania. The family seems to have been firmly respectable (his brother is an oncologist) and, when Benicio graduated from high school, his father edged him towards business school in San Diego. Like so many actors, he stumbled across his eventual career almost by chance. Having taken the odd acting class, he contacted an agent in Los Angeles.

"Yeah, it was a series of accidents, my man. This agent said there is this acting school here in LA and they are offering a scholarship. So I go to this school and there is a picture of woman in this actorly pose on the wall: Stella Adler. I didn't know who she was. I did three monologues and they said you can have the scholarship."

At that stage, Adler, an early evangelist for Method acting and an important mentor to Marlon Brando, still kept a personal eye on the studio that bears her name. "One day she turned up and watched me and tore me up, but that was all good. I had the best teachers at the best school."

Sadly, despite his seething charisma and powerful intensity, Del Toro initially suffered from the usual prejudices that dogs Latino actors. For the first few years of his career, he played a few too many drug dealers, petty thieves and assassins. His break-out role at least allowed him to create a more interesting class of villain: the odd, mumbling, barely coherent Fred Fenster in Bryan Singer's The Usual Suspects.

"A lot of people think that was my first movie," he says. "It wasn't, but it was the first that they took seriously."

Still, I had read that, following his performance in Fear and Loathing in Las Vegas, he suffered another bout of stereotyping. In Terry Gilliam's adaptation of Hunter S Thompson's disreputable book, Benicio gave a terrific performance as Dr Gonzo, the narrator's pharmaceutically befuddled attorney. Is it true that, after Gonzo and Fenster, directors only wanted to cast him as weirdos?

"I don't think so. That's not quite true," he says. "I had always played odd guys. After The Usual SuspectsI actually got jobs. Everything was good. But what happened was I gained weight to play this drinker in Fear and Loathing in Las Vegasand I didn't get much work after that. Hunter predicted that. Rumours went around. 'Oh he's turned to booze. He's got fat.' Then Steven threw me the rope that was Traffic."

That's an interesting way of looking at it. So he did feel that his Oscar-winning turn as a Mexican police officer in Soderbergh's Traffic helped him out of a hole? "At the time it just felt like a rope, but, after the success, it felt more like a ladder I suppose."

What about winning the Oscar for Best Supporting Actor? Did he wake up as part of the establishment the following day? "It does feel good on the business side and it is good to have the respect of your peers. I suppose it helps you get a table in a restaurant as well. Though I don't carry it around with me, of course."

Del Toro hasn't worked too hard in the years since that win, but the roles he has taken have tended to be good ones. He was scary in Robert Rodriguez's Sin Cityand movingly heart-sore in Alejandro González Iñárritu's 21 Grams. Now he faces his biggest challenge with the massively tiring pincer movement that is The Argentineand Guerrilla.

It hardly needs to be said that Che Guevara is a controversial figure. Soderbergh's film adopts a cool - one might say arid - approach to his subject. The camera rarely indulges in a close-up. The dialogue, almost all of which is in Spanish, is delivered with a surprising lack of passion. Yet a thin whiff of hero worship does hang about the picture.

"Look, in a lot of ways I can't defend Che," Del Toro says. "He had people shot. He was pro-death penalty. But you have to remember he was a product of the 1960s. Think how times have changed. We are about to have a black guy walk into the White House. I don't think that would happen then. Here was Che at the United Nations saying to the USA: 'Don't lecture the world about democracy when you have these problems in your own country.'"

Another potential barrier to the film's popular advance is its unwillingness to fill in historical details. These two hypnotically focused films actually expect the viewer to have some knowledge of the Cuban revolution and a vague understanding of political tumults in Bolivia. How dare the film-makers not treat us like idiots?

"Well, that was Steven's choice," he says. "I may not have wanted to do that. But, yes, I don't like movies that explain things too much. I like films that expect you to know stuff. This film is more like jazz than a pop song. Think about the album Bitches Brewby Miles Davis. Play it to a 17-year-old and he'll scratch his head, but after a while he'll get it."

He's right. The Argentineis not a riot of energy in the style of The Battle of Algiers. It is an eccentrically modulated piece that works its magic on you slowly and deliberately. Odd as Che is, it might ultimately come to be seen as Soderbergh's best film.

"Thank you, man. I'm glad to hear you say that. Look, put that in your paper. Put that in your paper. We need the money." I am happy to oblige.

• The Argentine opens on January 2nd