1.) Prep in advance, gather and read through materials that you could, think of possible different angles, trajectories, and questions that could come up. Preparing your stand is important (but be mindful that you have to be flexible too as new information comes to light). 2.) I'd usually ask to give more time to it - say that I have to take this away to get a better appreciation of the matter, consult with so and so (e.g. another team or your line manager or an SME), consider other factors in the organisation, etc. The important thing is commit a timeline with them on when you'd be back with a decision.