With the unbridled success of Iron Man 3, on set to beat all expectations, it's clear that Marvel have found themselves a cash-cow in the shape of Robert Downey Jr. It's quite funny to think back to the first Iron Man. Jon Favreau, the director of the first two films, had to lobby hard for Downey's involvement who was seen as a bad choice for the role. Now, like Indiana Jones and Harrison Ford, you can't see one without the other. Which, in a way, is precisely why Robert Downey Jr. should leave Iron Man alone.

Downey Jr., in between Sherlock Holmes: A Game Of Shadows and Iron Man 2, completed a film with Joe Wright of Atonement fame called The Soloist. It was a fantastic drama that had Jamie Foxx, incredible music and a very emotional story. Nobody saw it. Why? Because it didn't have Downey Jr. suiting up and zipping around the screen. True, the subject matter of The Soloist might have been a bit heavy - it featured Foxx as a genius-level musician that's homeless - and the marketing of it probably didn't go according to plan. But still, Downey Jr. was excellent in and it didn't do huge business. Let's take another example - David Fincher's Zodiac. Downey Jr. was his usual wry, witty self in the role of hack-journalist Paul Avery.

Yet, when you think of Zodiac, you don't immediately remember that Downey Jr. was in it. True, Zodiac is an ensemble cast and it was more about the grisly murders than the individual characters. The point is, Downey Jr.'s involvement doesn't immediately spring to mind. You automatically associate him with Iron Man. It's not that his career is suffering under the weight of the role or that he's been typecast. It's just that he hasn't given himself a chance to do anything other than Iron Man. The only thing we've seen him in other than this was Sherlock Holmes - which was great and all, but again, he never had the chance to stretch out and, well, act.

If you look at his performances in Chaplin or Natural Born Killers, or even rom-com Only You, you can tell that Downey Jr. has the ability to act, really act. In the early 1990's, Downey Jr. was considered to be on par with Daniel Day-Lewis. Obviously, Downey Jr.'s well-documented problems with drugs forced him into a slide that he's now, thankfully, recovered from. If Downey Jr. was given a chance to act in a Paul Thomas Anderson film or a more substantial role in a David Fincher film, he could really be onto something. He's proven what he can do – especially with the likes of Kiss Kiss Bang Bang - given the right material and the right motivation.

So why should he finish up with Marvel? It gave him his career back, didn't it? There's a simple reason. It has run its course. (SPOILER ALERT!) Anybody who's seen Iron Man 3 will tell you that the film ends on a perfect note that beautifully wraps up his involvement with the franchise, without seeming forced or contrived. It felt, well, natural. The story had come to an end with a thought-out, calculated finish. Going back after Iron Man 3's finale would cheapen what came before. The story, thanks to Shane Black, was nicely cordoned off and to try and force it onwards when it doesn't need to is just bad writing and bad film-making.

Look at the Dark Knight trilogy. With The Dark Knight Rises, the series came to an end and it was done so with tact and grace. OK, they might have left it open for Joseph Gordon-Levitt to continue it on, but Batman has been reinvented in the past. In fact, it thrives on it. If you read Rory Cashin's piece on the subject, you'll see that Batman can and, no doubt, will be rebooted. Iron Man and Robert Downey Jr. are one. The two can't be separated – at least not any time soon. And with Iron Man 3 bringing the character's story to a close, it's best left there. Anything else would sully what's come before.