Character motivation is by definition the force that drives a character to act or react, which includes behavior actions, thoughts, or feelings. Action, resulting from motivation (reasons), can do several things. It can unleash activity; it can reveal character traits; it can reveal plot points; it can drive the conflict, etc. There may be active motivation or two kinds of passive motivation.
Active motivation results from the inner qualities of the character in question and is when the character acts. S/He is motivated to give her/is last few coins to a poor mother of two children because of a deep sense of compassion, as was the case more than once with Raskolnikov in Crime and Punishment. Active motivation reveals the inner qualities of the character and usually renders energetic action in the story.
Passive motivation results from external influences and is when a given character reacts, not acts. Passive motivation requires an influence to which the character can react. The influence may be either direct or indirect.
Direct influence is when something occurs directly to the character. For example, a brick falls from the top of a building and lands on Her easel, to which She reacts.
Indirect influence is when something occurs indirectly to the character. For example, She hears that Her brother's fiance is a jewel thief, to which She reacts.
In "Blues Ain't No Mockin Bird" the narrator is a passively motivated character who is reacting to indirect influences. The indirect influence to which she is reacting is the encroachment of the filmmakers on their quiet family life. The narrator's reaction is to take note of every detail that comes to her attention and write it in a narrative.
By way of contrast, Granny is also a passively motivated character, but she is reacting to direct influence: the filmmakers are attempting to persuade her; they are disregarding her; they are trespassing on her property and trampling her flowerbed.
The narrator is a young girl who is imaginative and intelligent. She truly loves her grandmother and grandfather who provide the family with strength and love. She's curious and perceptive. The narrator doesn't fully understand everything that happens, but this aspect of narration gives the reader an advantage. The narrator is innocent and unsophisticated, but she's also insightful. We are able to grasp the meanings even if the narrator doesn't always understand.