O'Neill is a charmer. Standing well over 6ft tall, with the air of the rock star about him, he explains what he thinks his job as costume designer consists of. He explains that he tries to get into a character's way of thinking, so that he can discover what influences these characters would have been under, what choices they would have made about what they wore. So, it's not just a case of thinking: "what did they wear in the 1860s? How did it look?", and then recreating it. This, O'Neill thinks, leads to a studied approach to costume design that can become "anal". "You end up trying to meet the expectations that you might get if you were doing a retrospective at the V & A, recreating the costumes from the 1860s and putting them in a display. For me, it's the essence of a period that has to be caught."
The costumes O'Neill creates are originals, either made from scratch or adapted from existing costumes from places like Angels & Bermans costumiers in London. Naturally O'Neill has to do an awful lot of research if he has to come up with something original, yet still exciting, for an audience that has by now "been fed period drama in huge doses". After reading and re-reading the novel and then the script, O'Neill then subsumes himself in tiny libraries, scouring their archives for ideas. He is most excited when he finds evidence of a person dressed in any way out of the accepted ordinary for that era. "It's like you've been given a licence by the someone of that era to go ahead and make something a little bit different," he explains. Thus, you can get English people in dress with a French flavour to it, influenced by something they might have seen on their travels abroad, just as we are today. "Frock coats were often black, but there were lots of paler colours around in France and because Dickens travelled to France and referred to it a bit, the country was relevant to that community, so I've shown its influence. I'll probably get lots of letters complaining, saying, 'they weren't actually like that, you know'," laughs O'Neill.
All this research means that O'Neill comes to the actual creation of a costume quite late in the process, after several weeks of "prep time". The costumes O'Neill has with him at our meeting are to be worn by Anna Friel, who plays the role of Bella. One dress is made from a pale blue taffeta, bought from a company that specialises in period fabrics (they are too expensive to be used for today's clothes), it is very tiny, showing how petite is Miss Friel, and very simple. "I'm not a decorative designer, I think things are much more powerful when they are simple. It's when you're losing your way with a costume that you can find yourself clutching at all kinds of straws - like putting lace on a skirt - if you can't get satisfied with it."
Simplicity does not come cheap with O'Neill's period costumes. Special fabrics like these are expensive and, together with all the corsetry and underskirts necessary to wear the dresses, the cost of just one of Bella's 13 or 14 costumes around pounds 3,000, all told. Despite this, there is a stain down the front, "Three thousand quid and she's spilled her butter down the front," tuts O'Neill.
O'Neill, together with his small team (who find the fabrics and the people who can then actually sew the garments in the short time available before filming) possesses an encylopaedic knowledge of places to find suitable contemporary accessories. Markets and antiques fairs are among the designer's regular haunts. Often, these contacts will make suggestions for each of O'Neill's characters and mutual trust is built up. He describes a trip to one such jeweller, in aid of Our Mutual Friend. He was looking for a wedding ring for one character ("a latter day lottery winner - he starts off as a dustman but ends up, via inheritance, as a lord"), so he enlisted the help of "some people" he knew who specialise in original period jewellery. They duly offered him the use of a gold and diamond encrusted wedding ring, costing pounds 23,000. So what did O'Neill do with it? "I popped it in my Tesco bag and left," he replies, a big grin creeping over his face.
"Our Mutual Friend" Monday 9 March, 9pm, BBC2