It was said in Prisoner of Azkaban that the Potters used Peter instead of Sirius as a Secret Keeper for the Fidelius Charm on their house because everyone would have expected them to use Sirius and so the secret would be better protected.
This made sense at the time, but Rowling has added some information about Secret Keepers in Pottermore that seems to cause a problem with this:
The Fidelius Charm is not without its weaknesses. If the Secret Keeper wishes to do so, they may divulge the information at any time (although the secret cannot be forced, bewitched or tortured out of a Secret Keeper who does not wish to give up their secret; it must be given voluntarily).
(Secret-keeper, Pottermore Book 3 Chapter 17 Moment 2)
If it cannot be forced, surely it didn't matter (protection-wise) if others knew who the Keeper was. Voldemort could have crucio-ed Sirius for months and he wouldn't get anywhere. Under these conditions, the only thing you need to consider to make someone a secret-keeper is that you trust them. Which means Sirius was a far better option than Peter.
One might think at first glance that they wanted to protect Sirius (as being tortured isn't a good thing whether or not it will reveal a secret) but since people thought he was the keeper anyway that's no protection for him.
So if Pottermore is correct, why would they pick Peter?
(I think Rowling messed up with that fact, personally. There was no reason to add it at all, as it doesn't help explain anything else I can think of)