The actor, who plays Tristan Farnon, has been missing since the beginning of series four, with his character having left Darrowby to serve in the Royal Army Veterinary Corps.
Chatting to HELLO! and other journalists at a recent press event ahead of the upcoming festive episode, Nicholas Ralph, Samuel West, Rachel Shenton and Anna Madeley spoke about the impact of Tristan's absence on their characters.
When asked if the cast had missed having Callum on set, Nicholas, who stars as James Herriot, replied: "Yes totally. As much as the characters miss Tristan, around the set we miss Callum."
Samuel, who portrays Siegfried, joked: "It's slightly quieter, isn't it?"
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Rachel quipped: "I get more cuddles from the animals because usually Cal is hogging that, so every cloud," adding: "No, I missed him."
On how the residents of Skeldale House are coping with Tristan being away, Nicholas said: "I think [for] James, Tristan is normally his best mate, he's like his brother in his surrogate home for the last few years. He took James under his wing to the point where [Tristan] was helping James out and he was starting to help Tristan out and they became this strong bond within the house.
"James misses him terribly but is also extremely proud of him for getting out from beneath Siegfried and becoming his own man."
Samuel added: "It's the same feeling for me, really. The whole of series one to three has been a process of discovering - to go back to the old cliché but true saying - if you love someone, set them free. As soon as I realised that and set him free, he left, which is exactly what he's meant to do but it hurts. I miss him terribly."
Anna chimed in: "I think Mrs Hall does as well. So she's enjoying having Carmody there, getting him sat at the table and feeding him."
Tristan isn't the only character away from home in the Christmas special, however. At the end of season four, James left Darrowby for the RAF training base, leaving his heavily pregnant wife Helen in the capable hands of Mrs Hall and Siegfried.
On the recurring theme of absence in the latest season and upcoming episode, Samuel said: "There is a lot of absence in this season. Obviously, absence is invisible but somebody told me, you know you're making something good when you're filming what's invisible and I think you can tell. Absence is shot through a lot of it and gives it a tone.
"Nic's absence is one of them that we really feel and you can sort of see it, even though he's not there."
So, what can fans expect from the festive episode? The full synopsis reads: "It's Christmas, and James is miles away at the RAF training base. An emotional phone call with a very pregnant Helen makes him determined to get back to her, but he doesn’t have many options and RAF Mascot, Georgie, needs his help. James struggles to do the right thing. Will he find his way home in time for Christmas?
"Meanwhile, Mrs Hall prepares a community Christmas celebration in Darrowby, whilst Siegfried and Carmody argue over who will be Father Christmas this year. Carmody is welcomed for his first Darrowby Christmas and finds himself enjoying it. James learns a very important lesson and is rewarded with a surprise."
The Christmas special arrives on Channel 5 on Thursday 21 December at 9pm.