That depends a lot on the movie. Apocalypse Now is sort of an extreme example of this: large parts of the movie (including the ending) were completely rewritten

That depends a lot on the movie. Apocalypse Now is sort of an extreme example of this: large parts of the movie (including the ending) were completely rewritten during filming, some scenes were completely improvised, the lead was recast from Harvey Keitel to Martin Sheen after they did about a week of shooting, an entire massive on-location set was relocated after being built because the director wanted it to be right on the river (and the original set was also destroyed in a typhoon), another set which was destroyed in the same typhoon was used as-is because Coppola liked how it looked, and the helicopters they were renting from the Philippine military occasionally flew off set in the middle of a filming day to fight an ongoing insurgency elsewhere in the country.