ad-lib, extemporize, extemporise, improvize, make it up as one goes along, play by ear, wing it, improvisate, improvisatize, freeball, more...

prepare, rehearse, plan

go off script,   think on your feet,   think quickly,   go off-script,   make up on the spot,   use what you have,   winged it,   to make do,   go off-book,   zero in acting