play by ear (?.)

• 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