Usually with my groups, we go directly to the meat of the game right after the character creation, but I see a lot of answers that basically say "you should have a session 0", but it's never explained what it is made of, so it's kind of an enigma for me.
Session 0 is a planning session where the gaming group collaboratively lays the groundwork for a new campaign. Often, this session involves the group deciding the game/campaign they want to play, managing expectations, establishing house rules, determining setting details, and creating characters. Session 0 provides a meeting for the gaming group to agree on what kind of game everyone wants to play.
Session 0 is done before the first session of play (hence the name). Some games designed as pick-up games, like Tenra Bansho Zero and Fate Core, actually have some Sessions 0 activities baked into typical play so you can start playing right away.
While it differs from game to game, Session 0 usually involves the group meeting to discuss and establish the following.
Session 0 is recommended because tabletop RPGs are ultimately a collaborative entertainment activity. Everyone is there to have fun and Session 0 gives the group an opportunity to establish what kind of game everyone wants to play.
Session 0 might not be necessary for some games and groups. For example, my groups usually work out these details in Discord or Skype before the first play session rather than schedule an entire session for it. I've seen some conduct Session 0 activities at a lunch. A well established group might not need a Session 0.
Roles depend on the game and group. From my experience, the GM is usually the individual responsible for spearheading topics and managing the session. From his interests and the interests of the players, he establishes a draft of a campaign and house rules to show to the players. The players provide feedback and then express what kind of game they are interested in. There's back and forth among the group before finally an agreement is made.
It's important to note that everyone should have a chance to provide input during Session 0. It generally should not be a one-sided affair.
The specific topics depend heavily on the game. Necessary topics in one group might not be necessary in a gaming group where everyone is playing a very familiar game and setting.
This reddit post for D&D 5E provides an excellent example of possible topics discussed in a Session 0.
My experience as a player is that a session 0 as far as I have experienced, is a session which normally happens before the main campaign which allows the DM and the players to meet and discuss the lore of the world and any house rules or homebrew content that the DM or players may want to use within the Campaign, it is also a time to discuss character concepts and the like.
This session basically makes sure that everyone, including the DM is on the same page and knows how the campaign is going to playout, whether it is going to be more serious in nature, NSFW or a more light hearted style of campaign.
This of course is all slightly subjective as each DM may run each session 0 different but it is a common trend in D&D and RPG games.
To answer your question as you have laid it out:
Before the start of the campaign
Normally the lore of the campaign, the homebrew and house rules and optional rules that wish to be used. The Character concepts if any.
Not Mandatory but highly reccomended. See my responses below.
To discuss the campaign.
Rules, Lore and Concepts of the campaign.
Because if you are playing any campaign that you are invested in, you want to spend more time playing rather than debating rules and lore.
- When is it done?
Before you start your game. Actually, normally even before you start your game preparation such as generating characters.
If you didn't have a Session 0 with that group already, it is actually never "too late" to have it, even if you are in the middle of your campaign already. Especially if you notice problems going on (which is very possible if you didn't have a Session 0 already).
- What does it contain?
It contains pre-game discussions of what are you all awaiting from the game, what is OK, what is not OK. Session 0 is (mostly) done out-of-character. The Same Page Tool is a good set of guidelines what exactly is it a good idea to discuss, but if you want to discuss something else too, do it!
An incomplete list of what do you probably need to discuss during a Session 0:
...and actually many more things!
- Is it recommended, if not mandatory?
If you are playing with a stable group of close friends, it is possibly not needed, you already understand intuitively what does each of you want. If you don't know all of your fellow players that well, it is more than recommended to avoid further misunderstanding. If you don't know people you play with at all, definitely have a Session 0.
If your players are completely new to role-playing, though, it is likely that they will just not know the answers to most question. In this case you better have it later when you try many things with them. After each of your gaming sessions you can then ask them what did they like and didn't like, which is actually never a bad idea to ask, no matter the experience level.
- What is each person's role (DM/GM, players, etc.)?
It depends. Sometimes GM serves like a "host" of the game and actually dictates what will the game be. You either stay and play as the GM says or opt out. Sometimes your roles don't really differ and you all discuss the game on equal rights, which makes you able to come to some conclusion together. Note that this conclusion may be that some player(s) doesn't (don't) fit in the group and has (have) to leave. It is really much better to understand it earlier than later, when you are already attached to your char and the plot.
If there are players of very different experience levels, perhaps the most experienced will lead the Session 0.
- What topics are best discussed there?
See p. 2, but again, the more you discuss, the less the chance to find harsh misunderstanding later. Don't be shy to bring up anything you want to bring up.
There is no full comprehensive list of those things. It cannot exist because of a huge variety of playstyles and potential problems.
If you had problems before, bring up what has caused them so they don't show up in your next game -- because you have solved them in advance, or because you found out that it's impossible and didn't waste your time on a game doomed to be unenjoyable.
"Session Zero" is, as the name implies, a session of the game that happens before you "really start" playing.
Session 0 is a kind of collaborative “character” creation session. While players may still have made many decisions about their characters prior to the session 0, session 0 would be a place to tweak those decisions, perhaps to avoid stepping on each others’ toes, or just to hash out how they know one another.
As the term “session” suggests, this isn’t purely done out-of-character. Roleplaying short scenes of shared backstory (even if it’s just meeting in a tavern) allows players to get a feel for one another’s characters, and it suggests that things are still at a more nebulous stage where changes can be made if there are issues to address or ideas to incorporate. As “session 0,” these changes aren’t even ret-cons, they are just ad hoc adjustments made during the session before things are in any way “set in stone.”
It also allows the players to get to know the world; NPCs can mention concepts, events, people, places, things, organizations, and so on that will be relevant to the story, or even not relevant to the main story but just facets of life in this world that the characters would be aware of and the players ought to be as well.
I recently ran a session 0 which I used to introduce the players to my world and then spent the majority of the session creating player characters with them. I felt like this was the best thing to do with 3 quite new players.
My advice is do it based on the experience of your player. If you try and wait until session 1 with new players I've found you spend 2/3rds of your time fixing up character sheets and rules details that can all be done ahead of time.
