As Star Trek embarked on its final season, creator Gene Roddenberry issued a memo directing the writers on how to approach the characters of Kirk, Spock, McCoy, and Chekov. It's a great peek into Roddenberry's management of the show.
Mission Log Podcast came across this memo, from April 1968. It's fascinating to see what Roddenberry values as he reevaluates his core trio of spacefarers and the newly promoted secondary character Chekov: competence, a respect for each individual's specialties, and a certain definition of manliness that involves the male characters respecting one another without being afraid to disagree (involves Kirk providing a strong father figure for Chekov). It's also neat to see how Roddenberry values fan feedback in his continued development of the characters (especially in light of a recent troublesome interaction involving a current steward of the franchise) and his tone in addressing his staff.
Document: 040 "Friday's Child" / Recurring Character Writing Notes [Mission Log Podcast via Giant Freakin Robot]