How well do you have to know the characters?
Very well. I have to get right under their skin. You start with the script and talking to the director, but I also try to imagine how the characters would be in their lives outside the script, so that as characters they “fit” their clothes.

How much research do you have to do to make sure everything is true to the period?
Oh, loads. You sit down with the director, the production designer, the props people and the cinematographer and research how the film will look.

How much input do actors have?
A lot. There is no point in putting them in something uncomfortable as it would detract from their performance.

On Mrs Doubtfire (1993) you created costumes for a man masquerading as a (much larger) woman. Was that difficult?
Well, I spoke to Robin (Williams) about how big he thought his alter ego should be, and the prosthesis he would wear was made. Then I got down to creating the costumes to fit. I based them on the dress of my old headmistress and my history teacher. It is exactly how they looked.

Costumes must take a great deal of wear and tear on set. How many versions of everything do you have to create?
For action films, many of them. You need several for the actor and the stuntmen. In Brokeback Mountain, the actors were in the saddle all the time, so the clothes took a lot of wear. Also, because the characters were from a poor background, each costume had to look worn to begin with. We washed things that shouldn’t be washed. Then we wet garments and hung them up with stones in their pockets to stretch things. And then dyed some bits to make things look faded.

How do you cope with having to repair costumes when scenes are shot many times?
When I worked on Ride with the Devil, a Civil War piece, one actor had to be shot in the arm. To get the effect, a little explosion fired bood through his frock coat. But they needed to film the scene six times and we only had three jackets. So we made extra arms with the left-over material and attached the new ones to the coat using Velcro in between takes.

Marit Allen is one of the exhibitors at Fashion Lives, British Library, 96 Euston Road, London NW1 (www.bl.uk 020-7412 7332), from Nov 11. Brokeback Mountain is on general release in Jan, 2006. Interview by Anita James