Executive Producer J. Michael Straczynski forced Michael O'Hare and Bruce Boxleitner to spend as much time together as possible (even making them share the same table during lunch) so that the characters exuded a believable friendship while on camera.
According to Babylon 5 canon, the scar on Sinclair's cheek was the result of a covert operation carried out while he was in charge of the Rangers.
Tim Choate's portrayal of Zathras is a highlight of every episode that features his character. Near the end of part one, Sinclair asks him if he is capable of transporting all the necessary equipment to the reactor core himself. "Yes, Zathras is used to being beast of burden to other people's needs. Very sad life. Probably have very sad death. But at least there is symmetry..." During the delivery of these lines, Mira Furlan (Delenn) turns her head to look at Sinclair, then back to Zathras, then down, and finally back to Sinclair, in a barely controlled effort to keep from laughing.
According to the Babylon 5 novel, To Dream in the City of Sorrows, Sheridan saved Sinclair from a mob during the Mars riots. The two later hid out in a bar where they got to know each other, and became friends.
This takes place from August 9 to August 12, 2260, in 2254 and on December 24, 2278.