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.

I think that Sirius suspected Remus because he was a werewolf and at the time, werewolves were siding with Voldemort. Remus suspected Sirius because his family had all sided with Voldemort and he could be the same.

I think that at that time everyone was very scared and no one knew who to trust. Especially in the last year where Dumbledore suspected that someone directly from the order was passing information to Voldermort, which I guess made everyone suspect each other. And if one thinks of it, it's pretty unnerving!

They suspected each other because they knew there was a spy working for Voldemort and neither of them thought Peter was sneaky enough to be the spy. Also they did not think that Peter could like them less than they liked him.

During Voldemort's reign of terror, there was so much uncertainty not knowing who to trust, I think this was the main reason.

I agree with that...

Because there was no body that they could really trust was there ??

I think they suspected each other because Peter was kind of a weak person. He followed them in everything they did, so they assumed he would follow in choosing a side of the war. I don't think closeness or being a werewolf had anything to do with that. They were still as close as brothers (as proven by the fact that Sirius never tells a soul about Remus's furry little problem, and Remus doesn't tell Dumbledore about Sirius being and animagus,) They just had no reason to suspect Peter.

Before the Potters decided to go into hiding, the Order knew that someone very close to the Potters was passing Voldemort information about them. Since it was said that the Potters escaped Voldemort three times beforehand, it was obvious someone was giving Voldemort the secrets. In that case, the only three close people to them were Sirius, Remus, and Peter. Both Sirius and Remus knew they weren't the spy, and they wouldn't expect the coward Peter to be capable of being a double agent as well. Sirius was so sure it couldn't be Peter that he suggested to switch Secret Keeper roles. Therefore, the only option left was for Remus to suspect Sirius, and Sirius to suspect Remus. After the murders, Remus was 100% sure it was Sirius, because he never knew of the switch (Sirius never told him, believing he was the spy). But Sirius knew immediately after the murders who the spy was, but he never had time or the chance to get the truth out there.

So basically, the fact that a close friend was passing info to Voldemort, and the tension that war causes were the factors for their mistrust of each other.

They may not have known each other all that well. James and Sirius knew each other very good,but the others just kind of tagged along.

Because they hadn't seen each other for over 12 years and everybody suspected black. What with him being in azkaban, there being many eye witnesses that saw him kill Peter and Peter being suspected dead for all those years. So I imagine Remus would have suspected him; that he thought that Sirius was not the man that he thought he was. And plus, nobody expected Peter to be behind lily and James' death because he was such a coward.

because peter was so weak i don't think either would believed james would use him as the spy both lupin and sirius were strong people and close to james each more likely to be chosen for secret keeper with more reason to accuse each other when it all went wrong

Remus and Sirius suspected each other as they hadn't saw each other in about 12 years. In that mount of time, feelings change. After all, Remus and Sirius weren't the best of friends as was James and Sirius so there may have been a bit of jealously there. But hey, who knows? Up to JKR to decide that.

I agree with those, who has said it was because Peter was considered weaker than the other marauders. Well, that was true actually. But they assumed that because of his weakness, Voldemort would never want to use him, that he would have preferred Sirius or Remus as his spy.

This is a really good reason. And also, in my favorite chapters in that book, Remus was never told about how Sirius made them change secret keepers.