Quote from PoA American Paperback pg 372-372

My question is - Why did Remus and Sirius suspect each other? From what I've read, they were aware that Peter was weak and liked to tag along with important people. Besides, they were BEST friends. Why would Sirius suspect Remus before Peter? Remus, I assume, suspected Black only after the Potter's were killed. Still, it seems odd to me...

I think ... well, neither of them were close, as close and James and Sirius (that is an understood, James and Sirius were apposed to be like brothers, knwing everything abou eachother, blah, blah), and I guess that they (Remus and Sirius) were never as close?
I never really thought of if, but it's a great question though.

Very well said dancrbookl! I think they also may have been slightly jealous of each other or the situation. It's like 2 best friends that what to be the best man at the wedding.

I think perhaps, Lupin had a crush on Lily and Sirius felt that maybe Lupin wouldn't do everything he could to save James...

I think Lupin might have been jealous of the realtionshp between James and Sirius and Lupin felt Sirius was a risky choice because he was a liittle off (Sirius did try to kill Snape)...

Maybe will find out in book 7.

i think as Lord Voldermort was at his most powerful, with so many wizards were under the imperius curse and noone knew whose side people were on. So there was bound to be alot of paranoia going around.Nobody knew who to trust so they kept important information to themselves.Unluckily, sirius made Pettigrew the potters secret keeper at the last minute. If he'd have stayed secret keeper maybe the potters wouldbe still alive.

i don't know why Sirius suspected Remus but, obviously, Remus didn't know about the doubts that Sirius were having when Lily and James made him their secret keeper...which in turn made him the biggest suspect for the betrayal...

I think Sirius was just confused coming from Azkaban and all...but now that I think about it, it is kind of weird isn't it? Sirius might have thought Remus was imperiused.

I think maybe it's because Remus was a werewolf. At the time, werewolves were on Voldemort's side, so Remus had a higher chance of being on Voldemort's side than Peter.

Sirius didnt know who to believe.
He also trusted Wormtail at that point because he always follwed them around and he thought that Wormtail would do anything for them.

I think there was a little paranoya there when Voldemort killed Lily and James. I think that anyone would have said anything so ppl would leave them alone about the whole thing.

Everyone was so sure that Sirius was guilty, he was locked up in Azkaban... Lupin was alone in thinking he was innocent, and after some time, perceiving Sirius as guilty probably came more easily, made more sense. With James dead, and supposedly Peter too, there seemed no other alternative than Sirius, right?

Besides, look at Black's family. They weren't exactly supportive of the Light side - despite knowing Sirius for years, Lupin could have seen this as a reason to doubt his friend too.

The fact, though, that he was merely suspicious yet not certain of Sirius' guilt, indicates that he didn't truly believe the accusations, just suspected them. He was being careful.

I think they suspected each other because they each knew that they themselves did not hold the secret, and neither would expect James and Lily to beholden a secret that large to Peter Pettigrew. So the only logical choice, in their minds, was each other. And after James and Lily were killed, they each, again, logically, suspected the other.