The Scot worked hard on his English accent before filming began - but Polanski was far from happy the first time McGregor spoke in character.
He tells WENN, "I spoke to Roman about playing the ghost with a Scottish accent and he said, no he had to be English. Also, he (character) went to Cambridge (University), so my first thought was that he would have a standard English accent; I find it difficult to do that accent without feeling kind of posh. I really wanted the ghost not to be posh.
"We were doing the wardrobe fittings and I hadn't read any of the scenes with Polanski and we had two days before we were ready to shoot. I'm trying on clothes and Roman appears every now and again and I said, 'Can we just talk, because if you don't like this accent I'm going to have to think of something else quick.'
"We finally sat down to read the scenes and he was much more picky about how I said the lines. Right from the get go he'd say, 'No, no, why would you read it like this?' And he'd take the whole script and read my lines for me until finally he said, 'Now you see!' I didn't know what I had done differently other than I was a little bit more frightened than I was a minute ago.
"The next morning I phoned up Roman from my car and literally read him some scenes using the London accent just to see if he thought it was OK and I wanted to double check. I actually double checked sneakily behind Roman's back with Robert Harris, the writer of the novel - and he said it was alright."
But McGregor only had to get his accent right; co-star James Belushi underwent a makeover.
The actor explains, "We shot Belushi for six hours with hair and, after six hours, Roman went, 'No, in the book he has no hair,' and they went away and shaved his head and we re-shot six hours worth of stuff with no hair!
"I didn't know how he (Polanski) worked, so I was slightly taken aback. He's a total perfectionist but I'm really fond of him. He was an extraordinary director to work with and a great collaborator and he keeps you on your toes. The first day we shot for 22 hours. I looked at him and thought, 'F**k, I've got four months of this!'"