BEN OKRI Star rating: ***** IAN McEWAN Star rating: *** Taking charge of the stage and using his lectern like a pulpit, Ben Okri, pictured, enthralled his listeners with a performance as warm, inspiring and spiritually uplifting as if he were an evangelical preacher intent on making converts. In an unusually penetrating question-and-answer session with his readers, the Booker Prize-winning Okri described his work as a writer in a manner that would be utterly alien to practitioners of the school of cruel and sadistic realism: "I don't want to amplify the amount of horrors in the world," he said. "I don't think my job is to do that. But I don't think my job is to sweeten the world. I tell the truth as I know it. I incline towards what I prefer, because I know the power of the world. I incline towards the light rather than the dark."

A declaratory reader, whose excerpts from his poetry and his new novel, Starbook, were delivered more as if they were gospel than fiction, Okri was a magnificent speaker, replying to a slew of intense questions with profundity and wit. Starbook's exceptionally dark subject matter is slavery. He described it not as a fairy tale, which label many critics have hung around its neck, but as "an allegory about an impossible pain, an impossible tragedy - a fairy tale with steel teeth!".

To read his fiction and poetry it might appear that Okri works on a higher plane than realism, so it was instructive to hear that as a Nigerian he was always conscious that his work had to be relevant. "The thread in African writing," he said, "is social justice. In my culture the writer was held accountable. They were not allowed to write love stories that were not connected to anything. The writer had to be as responsible as a politician, a banker, a teacher. You could be taken to task for being frivolous."

When he came to England he found a rather different environment, where writers could write about anything they liked. This freedom has brushed off on him to a degree, although he confessed that "still that injection of dealing with what's happening in the world will always be with me. That's where the steel teeth come from."

Asked to elaborate on a line in Starbook where he writes of an era "when imagination ruled the world", he launched into a heart-felt riff: "We live in a time when it would appear that facts rule the world. We live in a time when it would appear that things we can touch and feel rule the world. I think we live in a diminished time for that reason. We are more than the sum total of facts about us. One of the key things that makes us more than those facts is imagination." Unlike a revivalist meeting, none of us rushed on to the podium to kneel for his blessing, but after an hour of his intelligence and compassion, I think most of us were up there with him in spirit.

The concept of sparing the world further pain is one the fledgling Ian McEwan would have had trouble recognising. In a strangely lacklustre session, chaired by an ebullient Ian Rankin, McEwan recalled a reading he gave in Adelaide from an early draft of his devastating novel A Child In Time, about the disappearance of a child. "After reading the chapter where the child is abducted, the novelist Robert Stone suddenly got to his feet at the end and delivered a passionate speech: What is it with us novelists that we go for the worst thing?' ".

What's interesting about McEwan's work, however, is the mellowing of his imagination, from the stark and brutal horrors of the roasted cats, incest and dissected corpses of his early stories to the marinated middle-class miseries of his most recent novels. I suspect his description of his latest, On Chesil Beach, would have mortified his younger self: "There's no terrible crime committed, not even a terrible tragedy, more a missed opportunity. It's a novel of regrets."

"I find something very rich, novelistically, about innocence," he said. On Chesil Beach, which is set in 1962 (before "sex began"), is an account of a couple of virgins who come to mortal emotional grief on their wedding night.

The plot devices of McEwan's mature fiction are leagues away from the shock tactics of his youthful works, yet no less impactful. Possibly more so.

As he said: "The drama of crisis allows you to alter the drumbeat of your prose, and how you see how the crisis of drama can reveal character, and how it lodges in the memory. Human memory, I've slowly slowly discovered, is fabulously unreliable."

McEwan's quiet reflectiveness may not have made for the most scintillating public session, but when brought to the page, the results are unforgettable.