Dom Joly: On with my dodgy anorak and I'm ready to rumble

The first day of filming looms and I am getting ready to rumble. One of the main secrets of hidden camera is to be, well, hidden. I don't mean snuck behind a bush waiting for people. I mean wearing the right costume so that you can just blend into the background. Disguises can often be too comedy and easily spotted. As an example, I have to wear pyjamas and slippers in one scene. The natural instinct is to go all Wallace & Gromit with Victorian-style pyjamas and check slippers. I opted for plain black slippers and blue pyjamas.

I've just realised that this is a terrible example as I'm going to stick out on the street in whatever set of PJs and slippers I choose. A better example might be shoes. A lot of people I know still check out people's shoes and make snap judgements based on their choice. I tend to wear quite "urban" footwear. Once in the wardrobe department, however, my shoes were rapidly discarded in favour of a job lot from Clark's.

"If you want to disappear, wear Clark's shoes," said my make-up lady. I'm not sure that this is the slogan the company is looking for, but, if they want it, there it is. Clark's shoes are ridiculously comfortable but they all tend to have the same look – that once seen in the title sequence of The Bill.

For trousers to disappear in, the favourite choice was H&M or Marks & Spencer. I tried to grab a quite nice pair of jeans from M&S's "posh" collection. These were snatched away from me as being a bit showy. "They're all done by secret designers," said my wardrobe lady. I asked her what she meant.

"Big name designers do the work under another name. That's how they make some serious money." I was slightly confused. Surely they'd make even more money if they sold them under their own name in Marks & Sparks? "Yes, but it wouldn't be good for their image. That's why they do it in secret." I nodded as though I understood, but I didn't.

We were now on to anoraks. Three grey and plain-blue examples were produced. The first made me look like a trainspotter. The second gave me a touch of the paedophile. The third had Dennis Nilsen written all over it. I plumped for the trainspotter. I might have to pick the kids up from school after a shoot, and don't want to cause a stir.

It all felt a little like what I imagine happens when you agree to go on a witness-protection programme. Your old clothes are discarded while you are dressed up as something you're not. Clothes, wig, make-up, and, lastly, the "flair" – rings, bracelets, watches, weird glasses. Hey presto, welcome to your new life.

Most of my new characters look unremarkable. Mostly, they give me an idea of what I might have looked like had I gone into IT and moved to Kent. I actually hate wardrobe days as, unlike most British males, I don't like standing in a small room dressed only in my pants while three women walk around me staring. Weirdly, however, once I'm out on the streets and being filmed in my costumes, I feel incredibly relaxed, and really enjoy slipping into someone else's skin. I'm not quite sure why this is but I'm sure that there are some huge therapist's bills awaiting me in later life, along with some sort of explanation.