LAUGHTER
They say that as the Aurors took him away, Sirius Black was laughing.
For some reason, whenever there was a moment of true seriousness, Sirius Black was always struck with an acute bout of Siriusness. He thinks of the most inappropriate - or worse, the most awfully appropriate - things, and that makes him want to laugh. He's always been able to keep from laughing, though a smile will sometimes slip through.
When he and Remus caught James snogging Lily Evans in an empty classroom at midnight Seventh Year, all he could think despite this very monumental and potentially disastrous discovery was, So this is why his hair is such a mess when he comes back from midnight kitchen raids! He's been snogging his way out of detention!
When James pulled Sirius aside and said that he was planning on asking Lily to marry him, and would Sirius be his best man, Sirius simply thought, Not snogging his way out of detention after all, then.
Even when he was sixteen and watching his father blast him off the family tree, he merely wondered, Why don't I smell burning tapestry? I should ask Lily how to do that.
And when Peter, his friend, killed Lily and James and then blamed Sirius for it before blasting the street and a dozen Muggles apart, and Sirius realized that he was now, according to everyone important, a Dark Wizard and a Voldemort supporter and a murderer of best friends, he was struck again with an acute bout of Siriusness.
Wonder if I'll get back on the tree for this one?
He was just too tired. James was dead. His best friend, the man who'd thought enough of him to say, "Harry needs a godfather, Padfoot. Whaddya say?" was dead, and so was his wife, the girl Remus and Sirius had caught him snogging Seventh Year. Both dead. Peter was a traitor. He was tired of it all. Just so tired. Too tired to waste energy not laughing when something was funny. So he sat in the street and laughed, and when the Aurors came for him he went quietly, still laughing. He was even laughing in his mugshot.
But if you looked hard enough, you could see the tears rolling down his cheeks.