In film, there is very little time for an actor to regroup their emotions. They need to be able to zero in on these at the drop of a dime. Film actors often have to hurry and memorize new lines or pages of a scene that is written on the spot because the director decides to make last minute changes. Adapting to the new change is not hard if the actor knows the story rather than attempting to memorize the lines verbatim. If you know your character, know what happened in the past, then it is almost automatic to know how they would respond to these new circumstances, without much thought having to go into it.
