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In Harry Potter and the Prisoner of Azkaban chapter 19, when Peter Pettigrew is in a really tight situation, he is pleading to Remus Lupin this way.
‘Remus!’ Pettigrew squeaked, turning to Lupin instead, writhing imploringly in front of him. ‘You don't believe this … Wouldn't Sirius have told you they'd changed the plan?’
‘Not if he thought I was the spy, Peter,’ said Lupin. ‘I assume that's why you didn't tell me, Sirius?’ he said casually over Pettigrew's head.
‘Forgive me, Remus,’ said Black.
‘Not at all, Padfoot, old friend,’ said Lupin, who was now rolling up his sleeves.
(In that conversation changing the plan refers to using Peter as the Secret-Kepper instead of Sirius.)
Had Sirius really believed at some point that Lupin was the spy? Was Lupin suggesting it seriously that Sirius would believe that?
I have the impression that neither of them actually thought that. Lupin had been convinced that Peter was guilty from the point when he saw him in the map, and at this point they managed to convince Harry as well. Lupin may have had enough of Peter's pointless objections and wanted to get over with this conversation as fast as possible, which is why he said this, and Sirius went along. But other readers didn't seem to have perceived the dialog that way. What's the truth?