The first rule of improvisation is AGREE. Always agree and SAY YES. When you’re improvising, this means you are required to agree with whatever your partner has created. So if we’re improvising and I say, ‘Freeze, I have a gun,’ and you say, ‘That’s not a gun. It’s your finger. You’re pointing your finger at me,’ our improvised scene has ground to a halt. But if I say, ‘Freeze, I have a gun!’ and you say, ‘The gun I gave you for Christmas! You bastard!’ then we have started a scene because we have AGREED that my finger is in fact a Christmas gun.
― Tina Fey, Bossypants