Sirius looked around them, alert and wary, but they were alone for now. He nodded at his former-- friend's-- words. If Remus had a better spell, he would have used it. That... wasn't promising. Still, it was better than Sirius's options.

He lowered his voice. "I was ready to use the Shield Charm. I can't use anything offensive." Sirius didn't see any reason not to share his limitations with Remus and every reason to tell him. Whether or not Remus believed him-- well, Sirius had no control over that. But maybe it would put his mind at ease.