Well, even before the book was published in 2005 Johnny Depp had bought the rights to the book to be made into a feature length film. At the end of 2006, Depp has also hired D.V. DeVincentis (screenwriter of two films, Grosse Pointe Blank and High Fidelity), as the screenwriter for the film.
You have to wonder, what's holding up this project if this was initiated back in 2006? But the fact of the matter, I could see that this film could be a complete bust of a film if the characters are not written in an interesting and convincing way. But if done correctly there could be something magical on the screen that transcends the original text. It would be easy to see this become a modern Breakfast Club, mixing and matching a group of people who otherwise would never interact on normal conditions (except here they all share the common bond that they want to toss themselves off the top of a building in London).
In addition to strong story telling, casting of these four different characters opens up a world of possibilities.
Now, Johnny Depp is producing, but I don't think there is any way without significant re-writing that Depp could play a part as one of the four main characters. Age wise he might be able to play Martin Sharp, but Martin plays the part of a British morning talk show host.
To cast the part of Martin Sharp, there are endless possibilities, part of me pictures Martin Sharp being played by Michael Sheen (maybe it's the similar names, and maybe it's seeing Sheen play Tony Blair and David Frost). I think Sheen could shine in this role and provide a bit of diversity to his resume. My second choice is Ralph Fiennes, but he may be too sophisticated for this role.
The other character who thinks has a wide range of possibilities is the casting of the homely 51 year old mother of a disabled child named Maureen. Reading the book there is one person that I practically pictured as Maureen, and that is Imelda Staunton. Staunton seems to have the age, stature, and mastery of the subtle performance that would be important for this role. I picture her Vera Drake character brought to modern London.
There's also Jess, the crazy 18 year old her own baggage and impulsive nature, foul mouth, and straight-forward honesty. Depending on when (if) this film actually happens might change the eligibility of the character, but if I had to chose someone know I'd be interested in seeing what Evan Rachel Wood could do, particularly at playing British. I have a feeling she can bring the sassy, crazed, emotional, character that Jess brings, with finesse not as a comedic performance. Over the top takes skill.
Finally there is JJ, an American who's stuck in London delivering pizza's after a breakup with his British girlfriend and the break-up of his rock band. There's a variety of young American actors who I'm sure could fit the bill, but here's where I would want the casting director to also consider pulling from an American Idol finalist who didn't make the final cut. This role seems like the perfect role to find a young charismatic want-to-be rock star from American Idol and transition them into this role.
We'll see if A Long Way Down, like other Hornby books, get a finalized big screen treatment, but Johnny Depp, if you need some casting advise, please see above and let me know how it works out for you.