So I just got to the part where you can access the crystarium.  Should I focus each character on one role totally or do you earn enough CP to level your character through several roles?

max out each role first before moving on to the next, and focus on whatever their...first one was. Like commander for Light, Sentinel for Snow, etc
 
And when eventually you get access to all 6 roles on every character, I would advise to keep focusing on the 3 you will have by then and ignore the rest

I'd say always go for the role abilities and role levels first, then gradually fill out the Crystarium by getting all the lower value crystals. Don't leapfrog. If you do just a little bit of grinding, you should be able to keep your group at the top of all three of their roles through most of the game.

Yeah, each character has 3 main roles. Always get those ones up first because they're where the majority of your stats will come from. The other 3 roles will offer more stats, but definitely not as high as their mains. For instance I think the max stat increases for Lightning's Strength in the Sentinel, Saboteur and Synergist classes is 10. It's something around 50 in Ravager, Commando and Medic, so you'd want to increase those first.

Focus it for one character, for example for Lightning focus on Commando and for Sahz Ravanger. Of course you could choose such as making Sahz a Sabotuer instead of a Ravanger, its all up to you.
 
As long as you focus on one class for one character you will be fine. Of course once you have maxed them out you can give them sub-roles such as making Sahz a Ravanger and a medic.
 
Also try not to focus on medics too much, they are only really used for a panic button kinda thing, but still its good to have a well leveled medic.

I am still at the point where I haven't gotten all 6 roles yet, so I can still basically fill everyone's crystarium all the way. I'd probably say to just focus on the three.

" Focus it for one character, for example for Lightning focus on Commando and for Sahz Ravanger. Of course you could choose such as making Sahz a Sabotuer instead of a Ravanger, its all up to you.  As long as you focus on one class for one character you will be fine. Of course once you have maxed them out you can give them sub-roles such as making Sahz a Ravanger and a medic.  Also try not to focus on medics too much, they are only really used for a panic button kinda thing, but still its good to have a well leveled medic. "

lol, i just grind until i can level eveyones job to the max before the cystarium expands again. Im still on Chapter 9 though so my process might be invalid later on.

I focus on abilities first then increasing stats. Afterall what good is a healer with good stats if they don't have the spell to heal everybody at once. 

Usually i focused on abilities and primary roles for each character. 
 
Fang/Light/Vanille was my main party though, and they mostly focused on Commando/Ravager/Saboteur respectively as primary roles.   For secondary roles they focused on Sentinel/Commando/Medic respectively.  
 
Snow I did Sentinel primary, Commando secondary.  Hope I never bothered using, but was Medic/Synergist.  Sahz was Synergist/Ravager but I also didn't use much (Though I do love him).

Go for whatever abilities you'll need first from the 3 main roles they were given, then go for stats your characters need from any of the 3 main roles that would cost you the less, I always get the abilities along with the stats the character need, for example I always give Fang str/hp on the way while grabbing the abilities. You should be able to get close to capping all 3 roles before it expands, except the end of the game if you decide to skip everything, then you probably would be further away from capping the 3 main roles they were given.

You don't really have to worry that much as long as you stick with the 3 main roles, because all roles have linear paths that provide you with stats you'll probably need, and you have other characters to compensate each other, just grab whatever you think you need.