If you’re an experienced freelance writer, you know this already: Prospective clients make up their minds about you very quickly—by the sound of your voice over the telephone, and the intelligence of your conversation. You don’t pass muster in that first five minutes, you know there will be no subsequent face-to-face to determine if you fill the bill.
I can’t help you with the sound of your voice, but you should know that they are listening for beneath-the-surface clues. Can they trust you to put words—the right words—in their or their boss’s mouths?
As for the conversation, the best way to sound intelligent is to say little. After the introductions are over, just shut up and listen. It is their dime after all, and you really need to hear what they are saying and what they are not. And frankly the more they talk, the more committed to you they become.
While you’re listening, you can take in other considerations:
They ask about money right off the bat. That means they don’t have much.
They have left the job to the last minute and expect you to bail them out of a jam. If you could only do a so-so job in a short time frame, turn it down and explain why. The good ones will appreciate your honesty and professionalism and think of you the next time. The others you don’t need.