For the special "From One Generation to the Next", specials directors Robert Meyer Burnett and Roger Lay, Jr. were not able to secure the cooperation of Spock actor Leonard Nimoy, so they approached the "cross-over" aspect of Star Trek: The Original Series into Star Trek: The Next Generation, from another angle by having interviewees discuss the earlier cross-over episode "Sarek" from the third season, whose main guest character, Sarek (played by the late Mark Lenard) also appeared at the start of "Unification I"