The leading role was taken by actor Christopher Timothy, after Simon Ward (who had played the part in the 1975 film), John Alderton (who had replaced Ward in the sequel, It Shouldn't Happen to a Vet) and Richard Beckinsale all turned it down. Siegfried Farnon was played by Robert Hardy. Tristan was played by Peter Davison. Helen was played by Carol Drinkwater in the first three series and two specials, then by Lynda Bellingham in the final four series. Mary Hignett played housekeeper Mrs Hall in the first three series, and Margaretta Scott appeared as the recurring aristocratic dog-owner Mrs Pumphrey.
With the amount of screen time to fill, the series quickly became much more of an ensemble show, developing all the characters considerably. In particular, the role of Tristan was significantly increased: partly because Christopher Timothy suffered a car accident part-way through the first series and so was restricted to studio scenes (in at least one episode, he can be seen having visible difficulty walking about the surgery), which required that all the scenes involving location filming be rewritten and given to Davison.(WIKIPEDIA)
The programme initially ran for three series, but broke off (in 1980) at the stage where the characters were drawn into the Second World War. This completed the adaptation of all the novels which Alf Wight had written up to that point.
Two 90 minute specials were subsequently made in 1983 and 1985, set after the war; and then in 1988 the programme was revived, when the BBC was able to persuade Alf Wight to allow new scripts to be written by other writers, and ran for four more series (taking the characters into the early 1950s).
In the revived series, Lynda Bellingham took over the role of Helen (following Carol Drinkwater's decision not to continue in the part, as a result of the ending of a real-life romance with Christopher Timothy), and Judy Wilson briefly played a new housekeeper, Mrs Greenlaw, as Mary Hignett had died shortly after the end of the third series. The Darrowby practice added a young Scottish vet with a liking for wildlife, in the person of John McGlynn, playing Calum Buchanan (based upon Herriot's real-life assistant Brian Nettleton), as a former classmate of Tristan's. The Herriot children, who had been introduced in the two specials, became recurring characters, with Jimmy played by Oliver Wilson and Rosie by Rebecca Smith.
The revived series gradually became a soap opera based around the development of the central characters - particularly after the introduction of Callum and Deirdre, with their romance and subsequent marriage - and it mainly centred upon the activities inside Skeldale House at Darrowby, rather than being a series about a veterinary practice. For the final series, all of the new characters were dropped (including Callum, Deirdre, Mrs Greenlaw and the Herriot children), and the series returned to its 1970s roots, focusing once more on the animals. The final broadcast was another Christmas special, in 1990.
In 2007, an unfilmed script by the show's script editor Johnny Byrne was recovered and presented to the BBC as a possible Christmas reunion episode, but the BBC did not commission it. Peter Davison joked, "Maybe they just thought we were too decrepit, I don't know!" (WIKIPEDIA)