Sara Rudner has created a four- hour dance piece, but she doesn't expect the audience to stick around for all of it, she tells Michael Seaver.

Making dance is a time-consuming business, so why create four hours of material? And then encourage audience members to come and go as they want? "Because it's what us dancers do," says choreographer Sara Rudner. "We dance."

Rudner is in Ireland setting her four-hour dance marathon, This Dancing Life, on a cast of 21 performers from New York and the Irish Modern Dance Theatre. And although the performance lasts for more than four hours she reckons every audience member should only stay for about an hour. "After that they get the idea," she says.

She is a veteran of the dance boom in 1970s New York, working in the fluid environment where groups came and went, but her association with Twyla Tharp is the strongest and she danced with Tharp's company for almost 20 years, creating signature roles in Short Stories, Nine Sinatra Songsand The Catherine Wheel. In this time she toured extensively and learned from performing far more than rehearsing. "A performance is worth 20 rehearsals," she says, adding that this generation of dancers haven't the same performing opportunities that came her way. This is one reason she "force-feeds" dancers hours of material. "But also, if you dance for four hours, you are pushing yourself to a place where you are exhausted and that's when technique becomes really important."

In 1975 she presented her first five-hour dance for four dancers. "I had a friend who came and left after about an hour. He then did his laundry and came back towards the end of the piece, and while he was doing his laundry he couldn't help thinking that dance was going on as he folded his shirts. He said that was a nice thought, 'out there dancers are dancing'. Those turning up to This Dancing Lifewill be given a schedule with timings, so whether you arrive at 7.15pm or 9.45pm you'll know what section you are watching. And Rudner wants to encourage people to come and go. "Sometimes they are reluctant to leave in case they miss something, but there really isn't any reason to sit through the whole thing."

For Rudner the dancer is the most important element, far more important than the choreographer. "I want the dance to be about the dancers. Choreographers might think of wonderful ideas, but they are useless without a dancer to realise them." She is eager to empower the performer, and strips away theatrical trappings, so the audience are "invited" into the dancers' space for the performance, the house lights will stay lit and if something goes wrong they can do it again.

This Dancing Lifereflects Rudner's life in dance, a distinguished career that includes creating work for Mikhail Baryshnikov, Paul Sanasardo Dance Company and Pilobolus Dance Theatre, as well as a reputation as one of the foremost dance educators. Although not overtly autobiographical, it is "about" dance. "This is a work about itself. It shows dance and dancing. So one section might be about flexion in the hip joint and although the audience mightn't understand the technique I really believe that if it's done well then they will understand the intention. It's like you don't need to know all the complexities of harmony to know the difference between dissonance and consonance."

SO DOES SHEhave systems to string out movement and fill four hours? Yes and no. Although she creates variations on movement patterns she is not a slave to a method. "I'm a formalist, not a rationalist. If I see a something I want to change I'll break out of the structure. There is always a danger of getting caught up in a system, where the system is what's being expressed. I use it to push me into areas I wouldn't normally go." Nor is she a minimalist, using constant repetition to fill up time. She doesn't seek solace in the predictability that comes with repetition, but rather within complexity. "I always look for calmness in complexity," she says. "I suppose it's because I'm a New Yorker. I don't leave New York for my meditation, but try to find it within the overcrowded city."

Although she won't micro-manage This Dancing Life, she'll fiddle with the work's loose structure during the performance. When she's not dancing (and her participation will be minimal) she'll sit beside a time-keeper calling cues and deciding how long each individual section will last. "During the performance we might rehearse difficult material before presenting it. I want the dancers to feel completely comfortable at all times. So I've told them we can rehearse the trickier bits and stop and repeat sections that might go wrong."

There are no notebooks or video tapes recording the material that is generated during rehearsals, just one body. Anneke Hansen, who works with Rudner at Sarah Lawrence College, is responsible for retaining all of the material. "We call her the somatic keeper. There are some dancers who have incredible muscular and intellectual memories and Anneke is one of them. She can retain tons of material and although others are also remembering moves and phrases, she does it more than anyone."

Known for her musicality as a dancer, Rudner considers music an equal partner in her collaborative process. "I think of dancing as visual music. I'm not that fond of setting dance to pre-existing music, because I believe that dance is a time-art and has it's own imperatives. It can be as subtle in its harmonic or polyphonic relationships as music can.

"When you are mickey-mousing the movement, in other words copying the music exactly, you are not being musical. Being musical is doing what music is able to do." The live music by William Catanzaro and Jerome Morris will include 78rpm records Rudner inherited after her mother died. "These songs by Glenn Miller and others have a particular length, so we'll often fit movement to them, but at other times the live music will continue what's on the record to extend and evolve into another section of choreography." How does she know she has finished creating? "When I get to four hours I will stop. But we will probably end up with nearly five hours material, because the dancers here in Ireland are so quick. If we reach four hours and we haven't used all of the material I'll ask their permission to extend it. If they say yes we'll go ahead, but if they say no, then that's fine."

• The world premiere ofThis Dancing Life is on Wed in the Carlton Ballroom, Kilkenny as part of the Kilkenny Arts Festival.