Is it true though that it's really hard to interpret idioms (e.g. 'It's raining cats and dogs') when signing?
Yes.
Interpreter friend of mine interpreted a lecture. Deaf students have an interpreter and a note taker, you can't take notes if you are watching an interpreter.
After the lecture the student asked my friend why she hadn't told her about the parrot. The student was quite angry that the parrot wasn't mentioned and it might have been important.
The lecturer had actually told them not to learn things parrot fashion, my friend had interpreted it along the lines of don't just learn the words, you need to know the meaning.
Little known (to hearing people) fact about BBC BSL interpreters. When BSL started being broadcast the BBC had a policy of using the same signs all the time (there are different signs used in different parts of the UK)
The original See Hear crew were mainly Scottish so a lot of signs used on the BBC until fairly recently were Scottish signs.
Sorry if this is a stupid question, but if they are deaf, how do they know what to sign? Do they use the subtitles?
Don't forget many programmes are pre recorded so the interpreter gets a script before hand. They also have an auto cue , some work from the English, others translate the English into BSL word order and have that on the auto cue too.
The interpreters on live programmes are hearing and interpreting on the spot.
If you want to see some fun interpreting take a trip to deafest
deaffest.co.uk/
On the Sunday evening there is usually a Disney or Pixar film that is interpreted, it's always one of the most popular parts of the weekend.