The Shutter Island star was shooting upcoming film Hugo Cabret on location with the director when she learned it was his birthday – and realised she had no gift for him.
Desperate, she remembered she had a birthday card intended for her godson in her luggage – and she decided to give him that.
The card in question featured a photo of British comedienne Catherine Tate’s seedy pensioner character Nan on the front with a spoken-word message inside.
Mortimer recalls, “I thought, ‘That is the most brilliant thing…’ (Tate) is brilliant but she dresses up as this horrible, wee-stained old lady with her knickers round her ankles.
“It was five o’clock in the morning and we were about to start the day, and I thought. ‘This is the most brilliant thing; I’m gonna give him this and it’ll be better than all those expensive presents because he’ll see the charm of the British sense of humour.
“It was a disaster… On the way in (to work) I was saying to (co-star) Sacha Baron-Cohen, ‘Do you think this is a good thing?’ And he was a bit like, ‘No, I don’t think you should.’ I was thinking he was jealous.
“But, as he (Scorsese) opened it I could see his little face fall… He’s actually a very classy guy and very refined… and this old granny with wee on her knickers… I could see him trying to be nice but it was just awful, and I couldn’t look him in the eye ever again.
“Sacha loved it – he tormented me the whole of the rest of the film.”