They didn't know
Warner Troyer: Did you know when you first outlined the series in your own mind, the concept that No. 1 was going to turn out to be you, to be No. 6?
Patrick McGoohan: No, I didn't. That's an interesting question.
Troyer: When did you find out?
McGoohan: When it got very close to the last episode and I hadn't written it yet. And I had to sit down this terrible day and write the last episode and I knew it wasn't going to be something out of James Bond, and in the back of my mind there was some parallel with the character Six and the No. 1 and the rest. And then, I didn't even know exactly 'til I was about the third through the script, the last script.
Troyer: How about you colleagues, the other writers. Were they surprised?
McGoohan: Yep..
Troyer: Were they annoyed?
McGoohan: No.
Troyer: Did they decide it was untidy?
McGoohan: No, they used to come along from time to time and say, "Who's No. 1?" you see. And I told them, "It's a secret" until I actually sat down and wrote it - and it was, actually; they didn't know until I handed out the script.
Troyer: But were they disappointed by that...?
McGoohan: No, they liked it. They said they always knew it was going to be him.