Choose your own Animagus?

[draco382]

No: HPFGUIDX 38419

Hi all,
I just have a general question about something I was confused about.  
Does it ever specify in in any of the books how certain individuals 
ended up with their respective animagus forms?  Specifically, was it 
implied in PoA that James Potter and Sirius Black CHOSE to be a Stag 
and Dog?  Or, does one end up with their animagus form by simply 
performing the spell and then waiting to see what happens?  For some 
reason, I always assumed that each wizard/witch chose the animal they 
were most fond of turning into.  For example, did Peter choose to be 
a rat because it was more inconspicuous?  Or do wizards turn into 
animagus (as people are suggesting) according to their 
personalities/abilities/life situations etc...
Sorry if this was already discussed.... :-)

~P