The Klutz: Paul Darrow, to a level that has probably not been matched until Matt Smith and a reputation that is legendary (and still growing). He broke every single gun he was handed, and the props department got so frustrated they made him one with an aluminum core so he couldn't break it... not to mention the sets and various buttons and levers.
One bts blooper runs like this:
Josette Simon in character as Dayna: "Avon, what are you doing?"
Darrow, the knob he was fiddling with breaking off in his hand, sending him into an adorkably abashed smile and fit of giggles: "Pulling the set apart."
The Other Darrin: Travis was played by Stephen Greif in the first season and Brian Croucher in the second. Less visibly, Orac's voice was provided in "Orac" by Derek Farr, who had played his creator Ensor, but in later episodes by Peter Tuddenham. The Federation official Ven Glynd was played in "The Way Back" by Robert James and in "Voice From the Past" by Richard Bebb.
Real-Life Relative: Paul Darrow's wife was asked to guest star on the show. She agreed on the condition that her character be killed by Avon because she found the idea amusing.
Talking to Himself: Peter Tuddenham, in scenes where Orac was talking with Zen or Slave. Tuddenham could reportedly do this with no need for ADR.
What Could Have Been: Terry Nation wanted the invading aliens in the series 2 finale to be the Daleks, and both Gareth Thomas and Tom Baker pushed for a 'Blake's 7/Doctor Who'' crossover where the Doctor and the crew meet up, but the BBC vetoed both ideas.
During a writer's strike Paul Darrow wrote an episode script for series D in which the crew desert Avon, marooning him, concentrated on Vila being more heroic than he had been for that series and also be the one who saves Avon by convincing the others to go back for him. Chris Boucher said no.
You Look Familiar - Glynis Barber also had a tiny role as a Mutoid in the first-season episode "Project Avalon", before getting the regular role of Soolin in the fourth season.