I have some Sirius questions here ... seriously :)

[find_sam at hotmail.com]

No: HPFGUIDX 12280

<snip> why 
> would (Sirius) laugh if he were innocent and witnessing Pettigrew 
actually 
> doing the murders?

Maybe Sirius WASN'T laughing at all, but howling with grief at what 
he'd realised just happened: two of his closest friends have been 
killed, and he's just been framed by the traitor. We learn later in 
PoA that laughing and crying can be easily mistook: the trio think 
they've heard Hagrid howling with grief about Buckbeak's death, but 
later learn that he was actually howling with laughter at the 
Hippogriff's rescue.

Sam Brown