Trisha Brown has spent much of this week at the Walker Art Center working with local dancers on the nuances of her choreography and preparing to perform a drawing tonight to launch an exhibition of her works on paper, So That the Audience Does Not Know Whether I Have Stopped Dancing. On Wednesday, in the Walker’s Medtronic Gallery, Brown coached a handful of Twin Cities dancers — Emily Johnson, Sally Rousse, Morgan Thorson and Galen Treuer among them — to work out the curves of Brown’s 1968 piece Planes. Brown’s own company performs April 25 at Northrop Auditorium.