An actual session 0 checklist
Organisation (the boring part)
- How will we schedule our campaign? Every Saturday 6pm?
- Will we run in person or online, where or on what platform?
- How long will our campaign be in sessions?
- If someone can’t come or is late, what do we do?
- Will we eat during our sessions? Snacks and booze?
Overview
- What do we hope to achieve in this campaign? Just have fun?
- How much power should I have? Can I change your backstory?
- What game system?
- What is the genre of our campaign? Action-adventure?
- How serious will our campaign be?
- How explicit will our campaign be? 18+?
- What are the lines of explicit or violent content we won’t cross?
- Are there specific things anyone would want to veto?
- Will romance and/or sexual content be included and how much?
- What will we do if someone gets uncomfortable about something?
- What are the 1-3 main themes of our campaign? Heroism?
- Vaguely, what will our setting be? Medieval fantasy?
- What will be secret or public information? Public backstories?
- What will be the balance of action vs combat vs roleplay vs intrigue?
- Will we use homebrew?
Party and PCs, all together
- What will the Party be? Fate-bound mercenaries?
- How much should we interject into others’ characters?
- One by one, what is your loose PC concept?
- One by one, what is your link with the PCs to your left and right?
- One by one, what is your concrete PC concept?
- How will you make your PCs work together story-wise?
- How will you make your PCs work together rules-wise? In combat?
- One by one, what is your PC rules-wise?
- How will the Party develop during the campaign? Developing trust?
Story
- In a few words, what will the campaign be about? Saving the world?
- What will be our specific setting? The city of Sharn from Eberron?
- Vaguely, what will the main villain(s) be like?
- What kind of NPCs should I include? Allies?
- Will there be side quests and how many?
- Will there be downtime and how much?
To each player individually
- What is your PC’s backstory?
- What is at least one friend and one enemy from your PC’s backstory?
- How does your PC’s story start? Looking for revenge?
- How will your PC develop during the campaign?
- How will or may other characters influence your PC’s development?
- Are there specific events you would want to happen in the story?
I've seen many of these so called "checklist", but they are always guidelines more than a checklist. So I made my own which is meant to be used verbatim by the DM, you know, like a checklist. I tried to make it as short as possible but still complete, with everything I didn't consider as absolutely common sense or too small a detail.
EDIT: added a few questions about boundaries at many commenters' request.