Nick Clegg was doing rather well in an interview this morning with Nicky Campbell until he was bowled an unexpected ball.
Give me one thing you like about Gordon Brown, asked Nicky. Not going to said Nick. It's not about personalities. Twice more, he was offered the chance to say something positive about the PM. Twice more he refused. OK, then said Nicky, say something nice about Labour values. Nick Clegg then proceeded to say how he used to think Labour had good intentions but now he doesn't. They then went through the same process about David Cameron.
By this time Nicky Campbell wasn't even trying to hide his contempt. "We've been sold a pig in a poke. I though the LibDems were all about positive politics." Clegg whined: "Well you pushed me into a corner."
And that's the difference between Clegg and Cameron. Cameron would never have allowed himself to be pushed like that. He would have answered the question in the first place.
It would have been so easy for Clegg to pay tribute to Gordon Brown's family life or something like that. He would have looked magnanimous. Instead he appeared petulant - something not unusual in Clegg interviews, as I have pointed out before.
If he really believes the election is not a beauty parade of three leaders, as he said in this morning's interview, why has he agreed to take part in TV debates with the other two leaders?
But there's a lesson for interviewers here too. If you ask politicians questions they expect, don't be surprised if they provide the expected answers. But the unexpected question will invariably provide a revealing response.