You get swept along by Williams's sheer love for his subject, and may well find yourself learning something along the way. Did you know, for example, that workers in hat factories in the 19th century treated the felt with their own urine? However, as many of them were taking mercury to cure syphilis, the fumes from that were slowly driving the workers insane. Hence, the phrase "mad as a hatter".
Williams conveys such facts with an almost evangelical sense of passion. To many, antique devices such as knitting-machines, cropping-frames, steam engines, water wheels and ploughs may appear to belong in some dusty, cobweb-strewn museum. But not to Williams. He is keen to bring them back to life, trying out as many of these ancient machines as he can.
"I just can't help being enthusiastic when I'm talking about this stuff," he says. "We take all these things for granted now, but I'm trying to conjure up the extraordinariness of what these industrial pioneers did. Think of the sheer bloody-mindedness of the people who went against conventional wisdom to construct the first iron steam ship or decided to level the land between London and Birmingham to build the railway."
On screen, Williams has the same eagerness as one of the puppies he starred opposite in 101 Dalmatians. It's here that his hands-on approach pays off. And when it comes to manual labour, he knows what he's talking about. In his time, he has worked as a waiter in a chip shop, a carpenter, a builder, an electrician, a gardener and - get this - an artificial inseminator of cattle.
"I want to show people what it feels like to use these machines," says the 46-year-old. "Otherwise, it's too arm's-length. You have to get your hands dirty in order to communicate. For instance, you can't work out what making paper feels like till you've done it yourself. So only when you have had to lean into a vat of slurry and pull up the mould against the huge water suction do you realise why all paper-makers had terrible backs."
The other reason for making the programme in this way was to avoid the "curse of the Open University": endless dreary talking-head shots of bearded academics plugging their latest books. "That's such a terrible cliché, and those documentaries are so slow," says Williams. "We thought we'd try to get in among it and involve viewers, rather than preach at them."
So what does the series teach us? "The human experience is that we often rely too heavily on the wrong things," says Williams. "The Victorians relied too heavily on coal, and they destroyed their cities and their children in the process. We're no different today. We've been the same organism since Neolithic times. We think we're cleverer than our forebears, but we're not. We still bring up our children in the same haphazard way, and if there is more than one switch, we still don't know which one turns the light on. The Americans spent millions of dollars trying to invent a pen that would work in space. Then the Russians went into orbit and found that the humble pencil worked perfectly well."
Brought up on a council estate in Bromsgrove, Worcestershire, he recalls proudly that when he won the school history award as a 13-year-old, he requested as a prize a copy of RA Buchanan's Industrial Archaeology - clearly a sign of things to come.
Now 44, he is an ardent Aston Villa fan, although, as he lives in Brighton, he has a fair way to travel for home games. He and his girlfriend Nicola have a young daughter. Williams is also a keen cook. He says that "actors always have to try to make ends meet - that's why they're often very inventive cooks. They wonder, 'What shall I do today? I know, I'll buy two pigeons because they're 50p at the market, and learn how to cook them.'"
Williams is perhaps best known for the sketches with Ken and Kenneth, the innuendo-prone, "Suit you!" tailors. The skit proved so popular that it has been revived for a mobile-phone advertising campaign. "Ken and Kenneth represent a kind of seething mass of horror we can all recognise," Williams says. "If they were allowed to say what they really wanted, all hell would be unleashed."
Williams went to Brasenose College, Oxford. But after graduation, he was not an overnight star. A man with pleasingly lived-in features, he says: "I think it took longer for me to succeed because I've got a face like the corner of a crocodile handbag. If I was a gorgeous young actress, it would have happened a lot quicker."
In fact, Williams has been making a name for himself in Hollywood. He has featured in Shakespeare in Love and The Borrowers, and will soon be seen opposite Steve Coogan and Rob Brydon in Michael Winterbottom's Tristram Shandy: A Cock and Bull Story.
But it is the Harry Potter movies, in which Williams portrays Ron's dad, Arthur Weasley, that have catapulted him to major-league status on the other side of the Atlantic. For his part in the films, the actor was obliged to make only one change to his appearance: "I had to grow my hair and dye it a real carroty red," Williams reveals. "I turned into a fuzzy-haired ginger person, and when Stephen Fry saw me, he said, 'Is that for professional or sexual reasons?' I quite like red. I think it made me look more bonkers." Williams is reprising the role in the forthcoming Harry Potter and the Goblet of Fire and will also be returning this autumn alongside Neil Morrissey and Claire Rushbrook in Simon Nye's BBC1 sitcom Carrie & Barry. He can hardly credit how busy he is right now. "If you'd told me when I was 25 that I'd be allowed to do a documentary series, a Winterbottom movie, a Harry Potter film, a Simon Nye sitcom and an advert all at the same time, I wouldn't have believed you," he says.
"I don't want anyone to suss me out, so I'm keeping my head down. It's like staying up late as kid - if you keep your mouth shut, you can watch telly for longer. That's why I'm not interested in fame for its own sake. You also have to work so hard to be famous, eating thin air, smearing yourself in St Tropez and changing your car every year. I'm quite happy with my camper van, thank you very much."
The actor is equally unfussed about the industry obsession with getting older. "I'm not bothered about ageing," he says. "All I care about is what I'm doing. My only career plan is: keep working. My mate, when I was a carpenter, used to say, 'Rule one: get up; rule two: go to work.' I have to make sure I don't blow it because I really like my job. I want to be working till I can't remember my lines any more."
'More Industrial Revelations with Mark Williams' starts on the Discovery Channel tonight at 8pmReuse content