But I think it's fair to say of David Miliband's antics last week that, while he might not have been wielding a knife, he let us know he had one, unsheathed it from its scabbard and held it up to glint in the sunlight.

The article in the Guardian that kicked off all the fuss might well have contained attacks on Brown for those inclined to read between the lines. But the lines themselves were pretty inspiring.

Society is not 'broken', Miliband argued. Crime has fallen more in the past 10 years than at any time in the past century.

His summary of Labour's achievements on health, women's and minority rights, employment and state-funded childcare were a welcome alternative to the defensive or despairing attitude we've been hearing from some Labour MPs lately.

His attacks on the Tories were also acute, painting them as pessimistic, vacuous and backward-looking.

Reading this stuff, I could see the appeal of jumping on board the Milibandwagon.

I got home and turned on the TV, and there he was again at a press conference, responding to the question of whether he was launching a leadership challenge.

Each time this question was put to him, he gave a startled, irritated shake of the head, as if a fly had buzzed into his ear.

The fly kept returning – as he must have known it would, since he put it there in the first place – and still he affected irritation.

It looked like deliberately bad acting. He had pretended to be annoyed by this ostensibly irrelevant question, while letting us know that it wasn't irrelevant at all.

Which was fine. This was no time for candour.

The problem, for me, came as he left the building and was approached by a journalist on his way out.

He was clearly high on the attention he'd just received, grinning from ear to ear, buzzing like the frontman of a sixth-form rock group on Battle of the Bands night.

The effect was compounded by his giddy response to flattering callers during his radio interview with Jeremy Vine.

Miliband might well be prime minister one day, which is a genuinely exciting prospect. But there were moments last week when we were reminded too much of how exciting it was, not for the country or the Labour Party, but for David Miliband.

We might find ourselves looking back on last week's performance, seeing him enjoying the attention that bit too much, and realising that his fatal flaw was staring us in the face all along.

"Vanity," grins Al Pacino's Satan in The Devil's Advocate, "my favourite sin..."

If you're going to show us your knife, don't do it with a grin on your face. It makes us doubt the solemnity with which that knife might one day be wielded.