How does the Fidelius Charm work?

[Lisa]

No: HPFGUIDX 23152

>I don't think the Fidelius Charm *removes* knowledge from
> people who already have it. I think it prevents people who *don't* 
>know from finding out.

Perhaps Sirius and others knew where Lily and James's house was 
already.  But only the secret keeper could tell others who didn't 
already know.  For example, if Sirius was forced by Voldemort to tell 
him where the Potters' house was, Voldemort would still be unable to 
find it.  Sirius could even have taken Voldemort by the hand and led 
him right to the front door, but unless the secret keeper had given 
Voldemort the directions, he would be unable to locate them.