I'm callin shize figs on no callouses.
I think that's an impossibility.
Sorry for necrobumping but i'm curious about the "learn how to scream/growl" thing and the "guitar player getting no callouses" thing :
-the "guitar player getting no callouses" thing : i was surprised about this also, and i also read Eyal Levi (producer, Daath guitar player...) say this also...
-"learn how to scream" : The Zen Of Screaming thing will not teach you how to scream, but how to master it/expand it/maintain it. It means you have to be able to scream a little (even if it kinda sucks and you don't master it) before watching it.
If you wanna learn how to scream :
1) as said before the biggest obstacle is the lack of direction and confidence when trying to sing with no loud music in the background, where you basically start screaming without knowing what sound you're trying to do, and then stop after 2 seconds because it sounded embarrassing.
-> Just put some music with screams you like and want to reproduce, relax and try to emulate them and record yourself.
Some will be in your range/comfort zone/tone of voice and be easy to replicate, and some won't and will take more work and time.
Try to find the right balance (and use some air out of your stomach to prevent your voice from fucking up) between pushing a little to produce enough energy/volume and pushing too hard (where you just damage your voice). If it starts to really hurt, then stop practicing for a day or two and come back at it.
2) before you practice those vocals, you'd better warm up. The Melissa Cross warm up exercises are awesome, and work for any type of vocals (clean, rap, growl, shriek, mid range scream....). Best would be to do them at least once a day, even if you don't intend sing for real that day.
3) as with every instrument (guitar, drums), you need power but you should not tense up too hard because it will fuck you up.
You should also relax and try not to thing too much about it when doing it, but just do it period.
The "audiating" thing in the Melissa Cross DVD is a good point and helps a ton in that regard.
I used to be able to scream a little (growls a la David Vincent, a little bit of shriek, and some mid range screams a la Burton Fear Factory / Jorge Merauder / Max Cavalera) when music was playing in the background and suck at clean vocals (mainly due to lack of practice and confidence), and i've been practicing (mostly warm-ups, clean singing (from low to high), rapping, and a little bit of pitched screaming/heat) for a few months and improved big time.
I somehow lost my ability to be able to scream spontaneously (which i could do but it would hurt quickly after a bit) in the process, and right now i'm kind of relearning it on a clean basis (proper warm-up + trying to sound right without damaging my voice at the same time).
I also learnt to imitate tons of different screams at very low volume, and getting them to sound right at real volume is another challenge and takes some work.
One of the most important ideas in the Zen of screaming thing is the pitched screaming part, where she says that when you wanna sing a raspy part in a pitch, just think of the pitch, and sing it clean, and then try to yell it (without screaming), and then try to scream it. One of the warmp-up excercises is great for that.
Also, something i wanted to master was really high crunchy vocals that ends up clean and high by the end of the vocal line (think most of the vocals in "best of you" by Foo Fighters, or the prechorus in Down's "Ghosts along the mississipi"), and i'm starting to nail it, by combining the "find the right notes first and sing it clean, then yell the notes individually, then try to scream while pitching it" technique + doing the "falsetto to high (non falsetto) note" warmup exercise (called "yee aa yee a yee a" or something like this).
I stil kinda suck, but it might post something here when i feel i sound decent enough.
Also, if you feel you can't go much further at screaming and get stuck, just be patient and also practice something else (clean vocals, rap...)... Cross training never hurts and allows you to keep things fun and fresh.
EDIT : Trying to imitate various types of vocals/screams at very quiet volume helps a lot also, especially for figuring out what's being sung right (words, pitch, tone...) and for figuring out how to shape your mouth and stuff like that. On the other end, it's not the end of it all, and it's another thing to get it to sound right when you use full volume (which is not necessarily super loud). Also, when you you imitate super quiet, it can also hurt since you're tempted to not use any "dump" (stomach air "airbag" so to speak), so don't use it too or for too long.
EDIT 2 : Growling is the type of scream that is more about nailing the right technique/intent and not going all out. It's about combining using a lot of air pressure (lot of air being pushed out), the right O shape in your mouth, and being able to go low (which takes some work/warm up/time depending on how low you wanna go) and sound raspy without pushing too hard. Growling is a lot about control of your voice, and not something that's natural and where you are letting things go. I'm pretty sure most growlers sound way quieter than screamers.
On the other hand, mid range screaming is closer to regular clean singing/yelling in terms of volume and technique. This is a bit more natural and you have to let go more.