Polanski's film stars Ewan McGregor, Pierce Brosnan and Kim Cattrall
Director Roman Polanski is finishing his latest film from jail, according to screenwriter Robert Harris.
Speaking at the Cheltenham Literature Festival, Harris said Polanski wanted to finish The Ghost in time for next February's Berlin Film Festival.
"He can make his wishes known from his cell. I don't think he can make phone calls but he can communicate," he said.
Polanski was arrested in Switzerland last month over his 1977 conviction for unlawful sex with a 13-year-old girl.
He left the US in 1978 before he could be sentenced and has never returned.
The Swiss Justice Ministry denied the 76-year-old bail last week, saying there was a "very high risk that he will flee".
Polanski's legal representatives are asking Switzerland's highest criminal court to free him and say they will fight any possible extradition to the US.
The Ghost is based upon Harris's best-selling political thriller which tells the story of a ghost-writer called in to help a former British Prime Minister write his memoirs.
It is set against the backdrop of a possible indictment at the International Criminal Court.
The movie stars Ewan McGregor, Pierce Brosnan and Kim Cattrall. Filming took place in Germany between February and May this year, and Polanski finished editing on the day of his arrest.
According to The Times, the director recently gave instructions about the film score to Alexandre Desplat, the composer best known for writing music for The Queen and The Curious Case of Benjamin Button.
Harris was promoting his latest novel, Lustrum, at Cheltenham
But Harris admitted the film's reception could be affected by the director's current situation.
"We will test to the upper limits the notion that there's no such thing as bad publicity," he said.
The arrest of Polanski, who won an Oscar in 2002 for The Pianist, has prompted an outcry among some politicians and Hollywood heavyweights.
But California Governor Arnold Schwarzenegger said the film-maker should not get special treatment because he is a "big-time movie director".
Harris - who also worked with Polanski on the script for a previous, abandoned adaptation of his historical novel, Pompeii - said he did not condone the director's crime, but felt the situation is more complex than some commentators have suggested.
"He was left with little choice but to flee," the writer said.
"He has effectively been on probation for 32 years. For this to happen to him now... it seems to me to be bad treatment, especially as the victim herself says that she doesn't want him to be pursued any further."