D'ailleurs vs. Au fait

Question

D'ailleurs, ça me
  rappelle que Paul m'avait prêté un livre palpitant. Can i use "Au fait" in this sentence instead of "D'ailleurs"? Thanks.

Accepted Answer

It's worth while to remember the three conjunctions "d'ailleurs", "au fait", and "à propos". All three chain two ideas together that are in some relationship. How close the two ideas are related is expressed by the conjunction used:
"d'ailleurs" talks about two ideas on the same subject."à propos" changes the subject but only to a related one."au fait" introduces a new subject

Answer

Hi Jenk,"By the way, that reminds me that Paul had lent me a thrilling book"Seems to me that your proposal is valid. There is the sense of "incidentally / by the way" in each of the two terms "d'ailleurs" and "au fait".So I feel that it would depend on the social atmosphere of the discourse at the time, but we cannot know that without being physically present during the discussion.In conclusion, I agree with you that substitution is possible.Let's see what others have to comment?Jim

Answer

Bonjour Jenk.
Chris's answer is very good and gives an excellent explanation as to why 'au fait' shouldn't be used in this sentence. Indeed, it implies that this idea / statement comes from 'nowhere' as it is used to introduced a new idea / subject. 
I hope this is helpful.
Bonne journée !