Fifteen years ago, Robert Downey Jr was sleeping on his assistant's sofa, addicted to heroin, cocaine and alcohol, and with only the first of several prison and rehab spells behind him. Even in Hollywood, where the most extreme appetites will be accommodated by the entertainment industry, his screw-ups had become too extravagant and numerous to hush up. His $2m asking-price had dropped 75% and he was virtually uninsurable.

Concerned friends described him as a "lovable tornado". Film-maker Mike Figgis, who directed Downey as an artist with Aids in 1997's One Night Stand, recalls him arriving for their introductory meeting barefoot, high and brandishing a handgun. "Why have you got a gun?" asked a concerned Figgis. "Oh, I didn't want to leave it in my car," Downey deadpanned.

Today he is perched at the top of Forbes magazine's list of the most highly paid actors in Hollywood. In the past year, he earned approximately $75m, most of it from one role. As the cocksure billionaire Tony Stark in the Iron Man movies, Downey brings a humanising, wiseacre wit to the wham-bam blockbuster formula, just as Johnny Depp did in the Pirates of the Caribbean series. Scandal aside, Depp is cut from the same snazzy cloth as Downey. Despite being stars, both men remain character actors drawn to kooks and oddballs. Their freakiness, so distinct from the glossy persona of a Will Smith or a Tom Cruise, should not be underestimated. (Remember, Downey Jr received an Oscar nomination for his crackpot turn in Tropic Thunder as a method actor who takes skin-darkening medication to play an African American. The character's name? Kirk Lazarus, almost too neat a fit for this back-from-the-grave showman.)

But it is his work as Stark in the $2bn-grossing Iron Man trilogy – not forgetting Avengers Assemble, responsible for adding another $1bn to the tally – that now defines him. It seems almost too obvious to mention charisma when the magic of his performance lies in making us feel that Stark's superpower has nothing to do with rocket-boosters and high-tech armour: it's his personality. Why else would the film-makers keep him out of that depersonalising tin-can costume as much as possible? Where some actors bring a functional blankness to superheroes (think Henry Cavill as Superman or Christian Bale as Batman), Downey dances and doodles all over the role, using it as the springboard for his standup-like flights of comic fancy.

The preening smugness he lends Stark draws on his own complacency in the years leading up to that protracted fall from grace. Not that his work now is contrite or humble – where would be the fun in a chastened Downey? But it is lived-in. He has tasted a world beyond privilege, and it shows. It used to be that one could look at Downey's CV and construct the story of his life using his movie titles alone. That's Adequate. The Last Party. Danger Zone. Now the same purpose is served by just one: Lucky You.