Ritualists are capable of a number of different roles, from providing defensive support and protection to buffing allies. A single Ritualist can defeat a formidable opponent with ease, however, they are very vulnerable to damage while performing their rituals and very rarely are seen fighting on the front-lines. Ritualist share one weakness: survivability. They have many powerful abilities, yet tend to not do very much damage themselves. Thus, they tend to be high priority targets. They tend to require a lot of protection, however, they more than make up for their vulnerability with the benefits their rituals bring to their party.
While performing the ritual, a Ritualist can do no other actions or it will interupt the spell itself and would have to be started again. To perform a ritual, the Ritualist is required to perform a specific set of actions in a specific order, be that drawing something, adding some form of magical component to the ritual or reading from a spellbook perhaps, to make it work. A variety of ways can be Roleplayed for this, so long as it suits the style of the Ritualist and makes sense. If this is disrupted, then the ritual is tainted and won't work, requiring the Rituliast to start again on performing the ritual.
A Ritualist has to maintain concentration while casting the ritual. If concentration is lost, they will have to start the ritual over again. This also means that you can't cast a ritual spell while maintaining concentration on another spell, ritual or otherwise.
Casting a ritual requires your action during any round in which you're casting it and should be roleplayed accordingly -- note that it doesn't require you to stay in one place, so you can move while casting your spell.
Note: When performing their rituals, Ritualists follow the same guidelines found in help rp-spellcasting as any other magic user.