accompany, ad-lib, be caught napping, be surprised, be taken unawares, be unprepared, chord, concertize, cook up, dash off, do, do offhand, execute, extemporize, fake, go off half-cocked, have no plan, improvisate, improvise, interpret, jury-rig, knock off, lash up, make music, make up, perform, play, render, scrap the plan, strike off, symphonize, throw off, throw together, toss off, toss out, vamp, whip up, whomp up, wing it