Hugh Grant wears 11 different ties during one scene (when the Prime Minister first meets Natalie at 10 Downing Street). The actor had a nap during a break and then he put on a wrong tie. When the crew had noticed it, they decided to "play with it".
Although Emma Thompson plays Hugh Grant's younger sister, in fact she is 17 months older than he.
The airport footage shown at the beginning and the end of the film shows real passengers at the Heathrow Airport in London. It was shot by a team of cameramen using hidden cams. Whenever they'd managed to record a scene suitable for the film, they asked the people involved for their permission to use it in the film. The entire shooting took a week.
In one of the scenes Aurelia, played by the Portuguese actress Lucia Moniz, strips down to her underwear and jumps into a lake. The crew had a 45-minutes long meeting to determine what colour of underwear should she wear.
The Downing Street scenes were shot entirely at Shepperton Studios (including exteriors), but during the pre-production Richard Curtis and production designer Jim Clay were allowed to take a two-hour tour of the real Prime Minister's residence. They were escorted by the then-Chancellor of the Exchequer Gordon Brown and security staff. They could not take any photos or make sketches, so the designer could use his memory as his sole source.