I’ve kept a sketchbook for over twenty years. Not the same one - I don’t draw that slowly. Over the years, they’ve piled up on the bookcase and become a scribbly, visual diary of life and travels. It’s also a reference, a place to doodle ideas and something to keep yourself amused when sitting sad and alone in a café somewhere.

A sketchbook is the cartoonist's best friend. Indeed, depending what you've been cartooning recently, it might be your only friend. It's also a reliable indicator of national characteristics and an interesting way to get into trouble.

When you first start using a sketchbook in public, it is difficult to avoid feeling self-conscious. But you’d be surprised at how little attention comes your way. Unless you set up an easel and wave brushes about, the chances are you won't attract attention. Most people will assume you’re writing a shopping list or you're a repressed poet, and will studiously ignore you. At least, that's the case in Britain where we're far too polite to inquire into aberrant social behaviour. If you sketch abroad, things are a little different .

A few years ago, I was sketching in a French mountain restaurant. I got involved in the drawing and when I looked up, my models had disappeared. I heard some Gallic muttering at my shoulder and discovered they were all behind me, passing artistic judgement. From the shaking of heads, I gathered it was the general consensus that I wasn't going to be any competition to Toulouse-Lautrec.

Similarly, in America you have to beat them off with a Rotring as everyone wants to see what you've drawn. Given what a number of Americans look like - and the cartoonist's stock in trade is exagerration - it can be dangerous to show them the final results, unless you've been extraordinarily diplomatic.

You do get the occasional remark in the UK. These can be a little dispiriting: "What's that meant to be?" "Have you ever thought of being an artist?" or "You could be quite good if you practised."

My favourite comment came from a waitress after she watched me scribbling away for several minutes.

"That's quite good," she said, "did you do that?"

"No, I'm channelling the unquiet spirit of Picasso."

A major danger is that your victim will see your drawing. I don't do caricatures and my cartoons don't necessarily resemble my victim. And drawings go wrong. You try telling that to someone who has just spotted you're drawing them. "You've given me a huge nose! My ears don't stick out! What are you playing at? You can't draw me looking like that, I'll sue!"

So next time you see someone in the corner of a café, trying to be unobtrusive, scribbling in a little book … just act British and pretend you haven't noticed.