Depends on the setting of the RP but the majoirty of the time we make our own charace=ters, via a character sheet which is given in the thread under the 'OOC' tab. It's not often that a roleplay is run using existing characters but it is a possibility. You can adapt your existing characters to an RP for sure, just as long as it suits whatever rules/guidelines they set but I often create a new character for every roleplay.