Watch one round happen a moment at a time, then read what each role can do. Every setting is the same game with different character names.
The leader chooses a team. Everyone votes yes or no on it — not just the people going. If it passes, only the people on the team play a card, face down: Succeed or Fail. Good players can only play Succeed. The cards are shuffled before they are turned over, so nobody learns who played what. Arguing about that is the whole game.
Good wins by getting three missions to succeed — unless the evil team then correctly guesses who Merlin is, which takes the game at the last second. Evil wins by making three missions fail, or by getting five teams in a row voted down.
You don't choose this: it is fixed by how many people are playing — roughly one in three — and so is the size of each mission team. With 7 or more players, mission 4 needs two Fail cards to fail rather than one.
Everything anyone knows for certain comes from one moment at the start: the evil players are shown each other, Merlin is shown the evil players but not Mordred, Percival is shown two names and told one of them is Merlin, and ordinary good players are shown nothing at all.