Given this two-part story's convoluted time travel storyline - which involves visits to 2254, 2258 (specifically during the events of the Season One episode "Babylon Squared") and 2277 - they are two separate versions of numerous characters in the various time periods depicted:
1. The case of the 2260 version of John Sheridan (Bruce Boxleitner) is perhaps the most complicated as he time shifts between Babylon 4 in 2254/2258 and Centauri Prime in 2277. It is unclear what his counterpart from 2254 was doing during the specific events in question. However, it is known that, by 2258, he was commanding the Omega class destroyer EAS Agamemnon, his assignment before being given command of Babylon 5 in "Points of Departure". When he is transported to 2277, he seems to temporarily merge with his counterpart from that era, meaning that there is only one John Sheridan in that era in spite of the incidence of time travel.
2. The 2260 version of Jeffrey Sinclair (Michael O'Hare) remains on and around Babylon 4 for the duration of its trips to 2254 and 2258. As with Sheridan, the exact whereabouts of his younger self in 2254 are unknown. During the events of this episode, the 2258 Sinclair is experiencing the events of "Babylon Squared". Initially, he is aboard Babylon 5, preparing to investigate the temporal anomaly created by Babylon 4's arrival in 2258. Following that, he travels to Babylon 4 with Michael Garibaldi (Jerry Doyle), where he meets Zathras (Tim Choate) for the first time from his perspective.
3. As is the case with Sinclair, the 2260 version of Delenn (Mira Furlan) remains on Babylon 4 while it is in 2254 and 2258. From numerous previous episodes, it is known that Delenn had already been member of the Grey Council for several years prior to 2254 and was presumably aboard their ship at the relevant time. The 2258 Delenn was meeting with the Grey Council during the events portrayed here. This was likewise depicted in "Babylon Squared".
4. While the 2260 version of Susan Ivanova (Claudia Christian) is aboard Babylon 4 in 2254 and 2258, her 2254 counterpart is serving as the executive officer of Station Io while her 2258 counterpart is aboard Babylon 5, coordinating the evacuation of Babylon 4 at the younger Sinclair's order, as was seen in "Babylon Squared".
5. The 2260 version of Lennier (Bill Mumy) remains on the White Star in 2254 and 2258 while his comrades are aboard Babylon 4. As with Sinclair and Sheridan, his whereabouts in 2254 are unknown. It can be reasonably assumed that the 2258 Lennier was aboard Babylon 5 during the events of "Babylon Squared". However, he was neither seen nor mentioned in that episode.
6. During the events of this episode, the 2260 version of Marcus Cole (Jason Carter) moves back and forth between Babylon 4 and the White Star several times while it is in both 2254 and 2258. Although not much is known about his life prior to joining the Rangers following the death of his brother, he states in numerous episodes that he had spent the vast majority of his life on the Arisia Mining Colony. This would make it likely that he was living there in 2254 and 2258.
7. Given that the 2260 version of Zathras claims to be 110 years old and to have served the Great Machine on Epsilon III for almost his entire life, it is logical to assume that his counterparts from 2254 and 2258 were doing exactly that during his trips back through time.

This episode marks the final appearance of Michael O'Hare (Jeffrey Sinclair) on the series.

Jerry Doyle (Michael Garibaldi) only appears in this episode in archive footage taken from "Babylon Squared".

This is the first episode of the series not to feature any scenes set aboard Babylon 5.