Performers are liable to go deeper into character the longer they are required to play the role. People tend to adopt some of the characteristics of the people around them and of their general environment - in this case a character portrayal - as time goes by. At the same time, some of their personal characteristics may 'leak' into their character role, unless care is taken. This is controlled though the ability to review material filmed or videotaped at an earlier stage of the production and by the member(s) of the production team who are responsible for continuity. Continuity is not just ensuring the position of a glass of water on a table or a piece of clothing worn by an actor is correct in shots separated by several weeks or months, but also in how the actors behave while in character throughout the production period of a film. It is not unknown for productions to complete filming and then require additional footage at a much later date, if it is decided that new scenes need to be added or existing scenes reshot. In such cases, performers must be able to return to their roles many months after they have finished and moved on to new roles. Actors are trained performers who are, by the nature of their chosen profession, good at assuming different roles as needed. Actors who are unable to perform under these conditions do not remain actors for very long.

Movies are always shot out of sequence. Shooting depends a lot on weather and scheduling. A talented actor will and does say in character quite effortlessly but some actors never get it. I work in the industry and Halley Berry ,on "Catwoman" was always in character.I also worked with Brittney Spears on "Crossraoads" and she was never in character.Usually if the director has very few takes in a scene it means that the acting will not improve.

Narcissism.

You might want to ask Sacha Baron Cohen that. He spent entire months in character as Borat. It made life easier for him as he is in nature a very introverted person and found he got along with people better as Borat. He enjoyed it, in some ways - not bothering to wear deoderant, not washing his suit, having a legimate excuse to wear a moustache, being an extreme extrovert. He wore Borat like a comfortable shirt, and I suppose that's the secret to keeping in character.