i've been wondering too, but about other singers,
one of the things i do:
record clean lead vocals,
record a second vocal track, with bandpass and some distortion
(i use the sansamp or joemeek vc8 for that)
and i compress the shit out of the distorted vocal,
mix it in so you can JUST notice it,
the distorted vocal should Not be louder than the other
of course it depends...blabla...but you mostly overdo it,,so be careful
thats also how i make telephone noises:
a bandpass, distortion, extreme compression
i've had lots of bad results with putting FX on the lead vocals,
i had better and more fun results with FX on a second vocal track,
not just a copy but a 'double tracked track,,,,u know'"
my 2 cents
ps...always mess with using multiple layered vocals,
do it unnoticed, put the extra vocals enough in the back so u can only
hear it a bit....just listen closely to marilyn mansons voice,
try to cound how many takes they mixed on his chorusses...
sometimes it sounds like 5 different voices with a different pitch...