"I love putting costumes on stage and seeing how the audience responds to them," she says. "We are allowing the people on stage to create something special which the public enjoy.
"And it is the challenge of actually producing what designers and directors want – the challenge of taking that design and making it and putting it on stage."
That moment in the spotlight may only be fleeting but every costume has to be perfect and will have taken hours of work.
"We like to see the designs as soon as possible," says Siân, "because the process can be very lengthy. The start point is when the designer turns up with a set of designs – and that could be around 14 months before the show is on stage.
"This autumn we are working with Marie-Jeanne Lecca on Moses in Egypt and William Tell. This is our third time of working together. The first time we did Mussorgsky's Khovanshchina and last year we did Berg's Lulu.
"When I spoke to Marie-Jeanne at the beginning of her Moses in Egypt design process she was talking about how it was going to be all colour. The colours are very bright and they will be in that colour from the tip of their toes to the top of their heads – even their faces will be coloured.
"Marie-Jeanne has very distinctive designs and she comes with some very specific ideas. All the images on the fabrics for Moses come from Chagall paintings – it's all about the swirls, the paintwork and the colour.
"She sent me reference books galore and gave us lots of Chagall images and we then needed to recreate these ideas on a three-dimensional person."
Although Marie-Jeanne is designing the costumes for two WNO operas which are touring together, the costumes will be poles apart in terms of inspiration, colour, shape and fabric.
"William Tell is very different from Moses," says Siân. "The colours start quite dark, almost drab, but then, because it's all about the characters being repressed and gradually gaining freedom, they gradually get some colour in their Swiss elements.
"The clothes and fabrics are very utility with some screen printed and others digitally printed so they are different. Marie-Jeanne has a pretty clear idea of what she wants, a kind of work-wear, calico petticoats and things but they need to be in fabrics that will hold the dye and then the prints.
"In a perfect world we would have so much money we could have everything made just for us but that is not the reality and it's all about finding a way to create everything within your budgets. For the soldiers' uniforms in William Tell we found a Chinese website where we bought repo uniforms. We then stripped them down to the bare minimum and created a whole new look with new buttons, collars and braid.
"Also for Tell, we are creating some boars' heads for some of the characters which are very intricate and made of painted fibre glass. We have to make sure they balance just right so that the characters can wear them and still sing, see the conductor and move around on stage. You have to make it work."
"And of course it's more than that because they could be wearing two or even more costumes in each show. And for the principals we could have a number of casts. So for example for Carmen this autumn we have three Carmens and understudies.
And for each show, you have to make sure that she looks as good as she possibly can."
Bearing in mind the team, which is made up of 10 full time and part time staff as well as freelancers, will be working on three seasons with around eight separate productions across the year, they are literally creating hundreds of costumes at any one time.
Because WNO are constantly creating new works, the costume team also needs to be on its toes in terms of recycling.
"Once the costumes are not needed for a show they live in our stores. We might hire out a production with the costumes or they may stay there. The props and costumes are on three floors. When a show is 'deaded', as we call it, then the costumes are up for grabs. So this Carmen uses pieces we have used in other previous productions such as previous Carmens or Traviatas. We then dye them or re-dye them or make other changes to them so they fit the designs for the new production.
"We've got a costume from our recent Così fan tutee which started life as a Flora dress in La traviata in the eighties, then we worked on the design again for the youth opera and the skirt was the mother's dress in The Red Kite (a Youth & Community production) and then in Così fan tutee we have a bearded lady and we used that skirt again by adding to it. A basic piece of costume can just go round and round.
"There are certain items which you see as an investment. So if we have tail suits made, we don't say they are specific for a show. They are stock items which can be used in other operas like Die Fledermaus and Eugene Onegin and La traviata
"And there are operas like La bohème, Carmen and Madam Butterfly which come round again and again. We have pans full of Butterfly costumes because each time a Butterfly comes she's a different shape and Butterfly has been around since 1978!"
"If we get to a point where we think we can't use something again then the youth opera may use it or they may feature in our youth and community projects," says Siân. "When it is deemed that we really can't use it again, we will hire it out. So a costume is never ever really dead."
As well as creating costumes from scratch, the team also ensures revivals are kept fresh.
"I enjoy doing revivals, especially if it's a show you really enjoyed and you like working with those people," Siân says. "We do sometimes make some changes to a show when we revive it, particularly if it has been hired out elsewhere and another company has made changes to them – we may keep those changes or do other alterations. Doing revivals is very different from new shows because you know how it all works together."
With the constant cycle of the seasons, Siân, who originally knew nothing of opera, has worked on tragedies and comedies set in every time and place – and she couldn't imagine herself in any other arena.
"It is such fun doing so many different shows and working with so many designers," she says. "And you never really lose that sense of pride at seeing your work on stage and so many people enjoying it. It makes the challenges all worthwhile."
WNO's autumn season at Birmingham Hippodrome features Carmen on Nov 19 and 20, Moses in Egypt on Nov 21 and William Tell on Nov 22. For ticket information call 0844 338 5000 or click on www.birminghamhippodrome.com
WNO also take their autumn programme to Cardiff, Oxford, Llandudno, Liverpool, Bristol and Southampton. Click on www.wno.org.uk for full tour details.