when famous people make a movie why do they have to memorize a script instead of making it theirself?

Question by Brandon: when famous people make a movie why do they have to memorize a script instead of making it theirself?
When famous people are making movies why do they have to memorize a script?? Why cant they just make on up as they go along? Or, let the director give them a tiny part of the script one at a time without them memorizing it?

Best answer:

Answer by steel6in2k
Because they don’t film a movie in the same way you see it. They do scenes that might occur at the same location, but in separate portions of the script, at the same time. It is not a linear process, so the actors have to memorize the lines for the scenes they are filming that day. It is difficult to ad-lib in that type of environment (though comedic actors tend to be able to do it much better).

Know better? Leave your own answer in the comments!