Check out this voiceover example from Little Children. This movie uses voiceover from an omnipresent narrator who’s not a character in the movie but provides context to what the characters each experience. There are many ways to get into character and actors often research the jobs, hobbies, or activities of the characters they play so they are able to put themselves in that frame of mind. Physical appearance can be important in creating a character as well, and actors will often lose or gain weight, or change their hair colour to physically create the character they are playing.
