Too good to look true: Drama-documentary is often accused of adjusting real life for movie thrills. The opposite is often the case, as John Lyttle discovered

Scriptwriter William Goldman calls them 'movie moments' - real-life incidents that 'won't play' on film because they shatter the special reality every movie must create. What audience, Goldman asks, would suspend disbelief for the following: a man decides he must speak to the richest, most famous, most protected woman in the world. He scales the castle walls that surround her, and although he's spotted by a guard, he blithely proceeds to ignore screaming security alarms, shimmy up a drainpipe, find an open window, and, by pure chance, enter the bedroom of the fabulous female (her personal sentry is walking the dogs). She awakes to confront the intruder and hastily buzzes for assistance. No answer. So . . . Michael Fagan and Queen Elizabeth II have a chat about life and unemployment.

'It's fabulous newspaper reading,' says Goldman, 'yet it has nothing to do with proper storytelling. If you handed it in as a screenplay, you would find yourself thrown out without ceremony.'

Screenwriter Michael Baker knows the feeling. His Disaster At Valdez, transmitted on BBC 1 tomorrow night, ventures into drama-documentary territory to tell of the 11 million gallon oil spill that devastated Prince William Sound on 23 March 1989. Baker is only too aware of the prevailing charges against the hybrid form: exaggeration, massaging facts into the contours of a conventional linear narrative, employing invented dialogue and downright lies to plug the gaps when no one knows what really happened (all accusations noisily levelled at Yorkshire TV's controversial Stalker enquiry recreation, Shoot to Kill).

Baker, too, expresses unease about venturing into the grey area between reportage and entertainment - the problems with libel and privacy laws, the morality of making a drama out of a crisis. But his particular problems run counter to traditional critical complaint. Baker actually found himself 'writing down', pruning and deleting to ensure that audiences wouldn't laugh out loud at the plain truth. The task was not, as the cynical might except, one of balancing journalistic values with showbiz sizzle, but what to do with facts that were already too good to look true.

'I simply couldn't include certain events. For instance, on the night of the grounding the Alyeska pipeline company was throwing a dinner to celebrate their tanker safety record . . . Honestly . . . After much discussion we decided to include the dinner, though some people thought it would seem, well, unbelievable.' What had to go, however, was the Valdez city council sub-committee meeting, also held on the same night, at which pollution specialist Ricki Ott talked down an open telephone line to inform the assembly, 'Gentlemen, it's not a question of if we have a spill in the sound, but when.' 'We could just hear the objections,' says Baker. ' 'Didn't that meeting happen days or weeks before?' In the end we thought it would undermine the dramatic tension of the opening sequence.'

The tantalising embarrassment of riches rolls on. Baker practically groans as he recounts the loss of the night- time trip out to the stricken tanker. 'The environmental agent Dan Lawn (played by John Heard) went out to the ship on a boat with a guardsman and a female able-seaman. In the pitch darkness she began to scream. Her hair had become caught in the fan that defrosted the boat's front screen. And she kept screaming. The men panicked a bit, and in the end they couldn't untangle her. They were forced to cut her hair off.' The symbolism - technology trashing natural beauty - paralleled the bigger picture with eerie accuracy. 'I wanted to retain it,' Baker admits. 'This hysterical incident against the full scale of the environmental horror; it's black comedy. On one level the whole story is black comedy. But it's also a dollars 4 million dollar BBC-American co-production and Americans don't like irony.'

They do adore sentimentality though, which is where Baker seems happy to have wielded the blue pencil. He recounts the next omission with barely disguised relish. 'At the press conference where it became clear that the spill was a serious disaster compounded by inadequate follow-up, a little boy, obviously dazed, walked over to the fishermen at the back of the hall and asked (Baker slips into cutesy Yank moppet mode) 'Are all the fish going to die?' Now, this can be vouched for. But think about the line - pure Disney. It's mood shattering, unreal.'

Nearly as unreal as discovering that Captain Joseph Hazelwood, the man who guided the Exxon Valdez on to the fateful reef, is now employed to teach maritime students the virtues of exemplary seamanship. Honest.

'Disaster at Valdez' is on BBC 1 tomorrow night at 9.25.