We were sitting at her big kitchen table in her exquisite house off Holland Park, but I hadn't realised the poor Duchess was such a touchy subject. Within seconds she'd sunk into a monosyllabic sulk. I told her I liked the interview, and she grunted: "Oh, right." When I added that the public would always remember her for it, she snapped over her coffee: "Well, that makes me sad. 'Cos I've been doing this 12 years."
Things went on in this vein, with her answers getting surlier and surlier ("Did you like Fergie?" "I did like her." "How did you get the interview?" "We just asked." "Do you think she should have given it to you?" "Yeah." "Why?" "'Cos I made her look sympathetic.") until I asked if she wanted to interview Princess Diana. Then she started ruffling up her feathers and waving her head and practically scratching the ground with suppressed irritation. She asked ominously two or three times: "Are we going to do my series or are we going to do royalty? Just out of curiosity?" But when I asked about Helen Mirren, whom she interviewed for her new series, she just stared at me and said: "I want to know what angle you're writing from. I'm just curious."
"I don't have an angle," I protested, but she was already zooming into the ring, beak poised. "Well, I'm an interviewer, and I interview people, and when people don't respond to me I can feel it pretty quickly." "And you're saying I'm not?" I asked. "Yeah. I'm going to stop this. Now." And with that she snapped off my tape recorder and stalked to the phone.
I knew her parents were sick and she'd been up the night before when her youngest had a "freakout", but I started to wonder what exactly was going on when she almost sobbed: "You piss me off. I've worked my rocks off for eight months and I've never done this before and you piss me off. Somebody from the BBC should be here. This is a stitch-up."
I sat in shock for the next few minutes while Wax called up Clive Tulloh, her producer, to get the BBC publicity girl's home number, then rang her and had a long conversation about how I pissed her off. The minute she got off that call, she fired off a couple more to Tulloh - this was all taking place at 9.30 on Saturday morning - and then shooed me upstairs. Meanwhile the TV director Ed Bye (her charming husband, who had let me in in his tiny waffle dressing gown) could be heard organising their children to go out.
When Wax finally came upstairs she had calmed down, and to her credit - and possibly the PR's - agreed to carry on. It turned out that when she's not being a nightmare she is likeable and fun. The 43-year-old is also deeply vulnerable and quite terrified of failure: she compared going in to her interviews to walking into a lion's den.
"I'm being filmed all the time. And they may go - well, they may do what I did." (She has the grace to laugh at this.) "You know, but I try to prevent that. And you can't go in their closet when they're big movie stars. You have to be really good friends with them before they allow it, otherwise you get kicked out and then the real bad thing is, they tell their friends and you're boycotted. And that's the real bad thing."
That is the Jewish neurotic side, but then she can airily dismiss celebrities altogether. I asked whether Pamela Anderson Lee actually had anything to say and Wax replied as if stating the obvious: "Well, she was just a farm girl before she did Baywatch. She hasn't had any life experience."
She was surprisingly honest when I asked if her celebrity status helped get the interviews. "They don't know who I am," she said at once. "But what I do have are the tapes. So we can say: `Look what I did with Goldie Hawn.' And Goldie Hawn will tell someone else and then, like, you get references and they'll call up and say, `She's okay.' So if I stitch one person up it's like you hit a... strike." And then? "Well, you can't do one. 'Cos you do one and it's - over. And I have too much to - lose."
Her comments on Sharon Stone offered another insight. Stone was "beautiful and smart", Wax observed. "Well, she doesn't think so. She talks all about how fat she feels. But that's not all she talks about. She has a way of making sentences so that you can hear she could be a writer. And then she looks good, so I think, God - this is terrible."
I said she didn't look so bad herself (which was true), but Wax said instantly: "Oh, I'm not in that terrain. You know. I mean that's the, you know, the high road." I say: "But wouldn't being so beautiful bring more..." "Trouble?" she anticipates. "Yes, probably, but then I wouldn't - mind. I wouldn't mind." And I realise that Wax doesn't just admire great beauty like the rest of us, she is personally hurt she hasn't got it.
She wants so much, yet she has so much already: a great career, gorgeous house, beautiful children and a notoriously nice husband. To her credit she apologised as I was leaving. "Sorry about that," she said. "I don't want to be pernickety. I don't mind if you say, `I saw the show and I don't like it'. Then we have somewhere to go. It's the editing too, 'cos I do all the editing. So it's like you just slogged away doing this. You don't know what anybody thinks and I could take you saying, `I thought that was a flat line'. Then I'd ask why, and we'd have a conversation."
So I say, in full expectation of being kicked out properly this time: "One thing I did notice was that you cut interviewees off when they're talking." But she agreed at once: "No, that's completely right, I do, it's because I get nervous." It was honest of her to say so, I replied. She said eagerly: "No, I do, I just think, `Oh God, they're not going to answer'. And I should really learn to wait it out." While she is telling me this I think she looks quite different without makeup. Her face is freckled, and softer, too.
Ruby Wax Meets... starts tomorrow on BBC1 at 10.10pm