This article is from the Red Dwarf FAQ, by Patrick M. Berry pat@interpath.com with numerous contributions by others.
In Series 7, schedule conflicts made it impossible to film more than four
episodes with both Craig Charles and Chris Barrie. Since Lister is the
central character of the show, the decision was made to do four episodes
without Rimmer. Rimmer appears as usual in "Tikka to Ride," departs in
"Stoke Me a Clipper," and appears in "Ouroboros" and "Blue" only as a
flashback or a virtual reality simulation. However, the character returns
in Series 8.
The original Kristine Kochanski, played by C.P. Grogan, is still dead. The
Kochanski played by Chloe Annett is from a parallel universe -- one in
which she, not Lister, was put in stasis for hiding a cat from the Captain.
She joined the crew in "OUROBOROS", at which point the character was
recast.
 
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