All regular Agora Speakers club meetings are structured, meaning they mostly follow the same flow and involve the same roles, in order to ensure a consistent experience for members regardless of which club they visit, and also because the format is tried-and-true and it works. However, clubs are also free to innovate, add other roles, change the ordering, etc. See the sections about the Club Meetings for a detailed description of how a meeting usually proceeds.
How is it decided who does which role?
Every member can fulfill any role he or she wishes, Agora Speakers does not impose restrictions or requirements on any of these. The general recommendation is that members sign up voluntarily for the roles they want for the next meeting(s), as long as it free.
However, clubs have freedom in deciding requirements for the roles. The most common limitation that one can find is the requirement that members that want to do Speech Projects also have to have volunteered before that meeting to other roles.
It is not recommended to repeat roles in consecutive meetings, in order to give everyone a chance to be in all roles and experience the unique challenges that each role poses. Furthermore, there's nothing more boring that a club in which the Grammarian, or the Timer, or the Meeting Leader is always the same person.
What happens if someone doesn't show up?
It is not "exactly" unheard-of for someone taking a role not to show up at a meeting.
If you're the one that is not going to be able to make it to the meeting, it's very recommended that you talk to some fellow members that you know will be attending and ask them if they can take your place. If you don't know anyone, talk to your Vice President, Education or the Meeting Leader for that meeting to warn them that you won't be able to attend.
At a meeting, if someone with a role doesn't show up without prior notice, it is in general the responsibility of the Meeting Leader to find a replacement.