Recently, the 41-year-old actor teamed up with acclaimed director David Fincher for Zodiac, a thriller about a San Francisco serial killer who terrorised the city in the 1960s and 1970s, and taunted police with his ciphers and letters. Downey Jr. plays Paul Avery, a criminal journalist obsessed with the case. Based on the Robert Graysmith book, Zodiac also stars Jake Gyllenhaal, Mark Ruftalo, Anthony Edwards and Brian Cox.

How much of a stretch was it to play a boozing, junkie of a journalist, given your problems in the past?

I never would have drunk those sissy drinks that Paul Avery likes, and I had to get myself geared up to say: "Give me an Aqua Velva" or whatever it was. I kept coming up with other narnes for the drink so it sounded funny at the least. I think Jake Gyllenhaal came up with Aqua Velva. I don't even think that was a generational thing. The answer is, I don't know. I'm an actor.

It must be nice to play a substance abuser instead of being one.

Absolutely!

Mark Ruftalo was saying how obsessive David Fincher was. Did you research your character?

I did some research. I don't want to say I didn't care. I had other concerns. Some people do a ton of research and then you're like: "Wow, that's really impressive. Your research is fantastic but you suck in the scenes." I would rather not suck in the scenes.

David Fincher is renowned for doing countless takes.

My preference would be to do two takes per day, but sometimes you've got to suck it up and sing for your supper 65 times. Fincher would call me over to the monitor and he would say: "Do you think we have it?" And I'd say: "Flawless!" And then he goes: "Yeah, but you see how Mark's leg dropped the thing out of the frame, what am I going to do, move my camera?" He would go: "Erase takes 1 through 65, we'll start over." Instead of "don't print", he's like "delete". "Delete act one, we can do better."

You'll be doing the film version of Stan Lee's comic book, Iron Man. How did that come about?

I went after it. I hunted it down. I am a fan of the comic book. It's mythology. Stan Lee created the character on a dare to see if he could make a likeable hero out of a wealthy, weapons manufacturing, hard drinking, womanising prick. I've definitely had to overcome certain perceptions. But I know underneath it, my heart is in the right place and that's kind of what the movie is about. Unlike so many other superhero movies where something amazing happens to them or they were traumatised by bats or something, which justifies their transformation, he's just this guy who has to do extraordinary things. That's why I love Die Hard and Matrix. I am a big nerd for big action and superhero movies.

You've received numerous nominations and awards over the years. Are your movie choices about winning awards?

Honestly, every time I have some motive, it doesn't pan out. I want Iron Man to be a massive success because it's a really good movie and it's marketed properly. I often buy into the trailer and the hype, and then I see the movie and think: "They just ripped me off. That was ghastly."

Many of your peers have called you the greatest actor of your generation. Is that a burden?

I'm fine with that. Look, I am 41. I've really got time; I'm going to make you proud. I'm going to live up to the title of "The greatest actor of my generation". It's like that final scene in Saving Private Ryan when Tom Hanks turns to Matt Damon and says: "Earn this!"