The surprise guest to open this 25th birthday edition of the Edinburgh international book festival was Prime Minister Gordon Brown, interviewed by Ian Rankin: an event so unutterably dull, flat and uninspired that I can barely even muster myself to report on it here.

What a difference a year makes. I first heard Brown speak at the Guardian Hay festival in May 2007, when he was about-to-be, but not-yet, PM. With a spring in his step and a glint in his eye, this was a man full of energy and promise. It helped, perhaps, that he was in conversation with Mariella Frostrup - a fantastically soft interviewer, but one who, with her cheeky and even slightly flirtatious patter, brightened up Gordon no end.

Today, however, the interviewer's chair was occupied by Rankin - a man whose writing I admire enormously, but one who is characterised by dry flashes of wit rather than sparkle. In short, here was a couple of downbeat blokes from the east of Scotland having a chat...

Then there was the fact that the anecdotes were old and tired. There was the one about how Gordon had spent much of his first year at Edinburgh University in hospital after a rugby accident: every evening at 8pm a trolley would come round bearing Guinness and beer. He likened the experience to that of Mark Twain, who grew up in a rural community and then travelled to Nevada. He wrote: 'This was no place for a puritan, and I did not long remain one.'" Well, I have heard that one before, and even written it up.

And - from a man whom Rankin, I am sure rightly, described as the best-read Prime Minister we have had for some time - there were curious slips. John Knox was described as 17th century; the setting of John Buchan's The 39 Steps was oddly confused (Buchan was, incidentally, another son of a Kirkcaldy manse).

Brown was ostensibly talking about his books on heroism and courage. I admire his insistence that true courage is not based on fearlessness but on the dogged determination to overcome difficulties to follow an ideal. But somehow he made it sound so very joyless.

At Hay last year, Brown seemed to be ushering in a new seriousness - promising a Britain that was to be based on firm moral values and social justice (the values of that Kirkcaldy manse, even). The fact that he was appearing at a literary festival - at its best, a forum for ideas and intellectual debate - seemed a great gesture. This morning, by contrast, it just seemed frivolous to be talking about his own literary output while, well, Rome burned.