A while ago, I was determined not to write a play for a well-known theatre. Just before going to see the artistic director, I ran into the designer Bob Crowley in the bar. "I'm going upstairs," I said firmly, "to tell him I'm not going to do it. I don't have the time and anyway, I don't want to." Next day, Bob rang me. "You know that play you're not going to write?" he asked. "Well, I'm designing it."

To this day, I'm not sure what happened when I went into the room. I took one look and before a word had been spoken, I knew I was lost. Later, I recalled James Carville, Bill Clinton's Southern lieutenant: "Once you're asked, you're fucked."

So much attention has recently been concentrated on the fortunes of battle that nobody has been ready to address the likelihood that, when the smoke clears, we will probably wake up with a new prime minister. Yes, the election campaign will have failed to address Afghanistan, Iraq, collusion in torture, climate change, the future of Europe and the collapse of the capitalist system. But tomorrow that much will be history. Instead we're going to be asking: what sort of prime minister will David Cameron be? If he can't even seal the deal, how on earth is he going to implement it?

All leadership depends on the defining ability to persuade people to do things they don't want to do. My first sighting of Cameron was in his shirtsleeves, doing one of his quick-fire question-and-answer sessions with 250 students in Brighton. Afterwards I did a straw poll outside on the lawn. The ones who went in liking him came out liking him; the ones who weren't sure still weren't sure; and the ones who hadn't liked him still didn't. After 45 minutes, he hadn't changed a single mind.

In his punishingly boring 410-page book of interviews with the editor of GQ, Cameron goes on record as saying that his favourite karaoke song is A Hard Day's Night "because I find it's better to sing something old, something familiar and something fairly easy to sing". But if, as he promises, he plans to inflict the most savage public service cuts in history, he will soon be singing a very difficult song indeed. An entire population will be asked to act against their own immediate interests. Nobody has yet observed that convincing them is a task for which Cameron does not seem cut out. As one Tory MP confided to me: "He's not the kind of person who, if you've suffered a misfortune, is going to put his arm round you and say: 'Bad luck, old chap.'"

After Hillary Clinton had been insulted by Obama in a candidates' debates, she had no desire to serve under him. But as soon as she was called to the White House, she knew she had to be his secretary of state. And a remarkable success she's making of it. She has even begun to move US Middle East policy away from the madness of Jerusalem and closer to the wisdom of Tel Aviv and of Ramallah.

Clinton took the job against her own instincts because, finally, when you say "I have to do it for Barack," it sounds convincing. It makes sense. So does "I have to do it for Maggie." And so, for a while, did "I have to do it for Tony."

But nobody in their right mind will say "I have to do it for Dave." Why not? Because you don't believe he would ever do anything for you.