No one really assigns roles or anything. Occasionally someone will ask you to freeze a big baddie or to use gifoie to stop chargers, etc. A lot of the people playing already know how and when to CC and support for the most part, so it is hardly talked about. If you are looking for more specifics on how to play, like combos to use, techs to use, useful status effects on certain monsters or whatever, make sure to be specific or people will probably tell you to just kill it
On my RAmarl I freeze the annoying things, use demon's on the big things, cast support/cc techs when there is no FO, and then just kill stuff.
Aside from quests like 1C and 2C, it's just a pretty chill do-what-you-like session. But aside from the obvious class roles I usually enjoy freezing/paralyzing enemies as my RAcast as long as the party can do enough damage.
I like to play support with my RAmar, but I don't like the typical heal slave role of supports.
I use my arrest needle to stun, demon's needle to weaken, and snow queen to freeze. I also use gifoie when needed in episode 4 to keep dorphons at bay. I love classes or roles that are more of a utility type.