THESAURUS
go off half cocked
ad-lib, anticipate, be caught napping, be predisposed, be surprised, be taken unawares, be unprepared, blow it, butt in, dive into, extemporize, foreglimpse, forejudge, forerun, foresee, forestall, foretaste, get ahead of, go before, have no plan, improvise, interrupt, intrude, judge beforehand, jump the gun, play by ear, plunge, plunge headlong, plunge into, preconceive, preconclude, predecide, predetermine, prejudge, presume, presuppose, presurmise, rush into, speak inopportunely, speak too late, win the start
