This one is quick & good (and helpful when you think of character dialog): According to Schrampfer Azar, œThe use of shall with I or we to express future time is possible but uncommon in American English.  Shall is used more frequently in British English than in American English.

UK: I shall go to the fair tomorrow.

USA: I will go to the fair tomorrow. (or more exact, I’ll go to the fair tomorrow).