How do I speak confidently on behalf of my boss?

Speaking for your executive intimidates you because their name carries authority you are not sure you are allowed to wield. But remember why you are in this seat: they trust you to be their voice. That trust is not an accident. It is a judgment they made about you, and it is worth believing. The confidence you are looking for comes from clarity, not from borrowed status. When you know what your executive actually wants and how they think, you are not guessing or pretending. You are relaying with authority because you truly understand the intent behind the words. The intimidation shrinks as your certainty about their mind grows. So invest in knowing your executive deeply, ask how they would handle situations, learn their priorities and their lines. When you are unsure, confirm before you speak rather than freezing. Over time you will represent them almost as they would themselves. You are not impersonating authority. You are carrying a trust that was deliberately given to you.