Smith-Bynoe stars opposite Charlotte Ritchie and Lolly Adefope in the hit sitcom, which began in April 2019.
The series follows a couple who move into a country house already inhabited by a number of ghosts.
On Tuesday (6 December), the official Twitter account for BBC Press office shared a post promoting the forthcoming Christmas special episode.
The announcement was accompanied with a promotional picture that showed the cast members taking a festive-themed photo.
In the centre are Ritchie and Smith-Bynoe in character as Alison and Mike Cooper, sitting on a red sofa.
Smith-Bynoe shared his reaction to the photograph on Twitter, asking: “WHAT have they done to my mouth in this picture?”
Fans have since reacted to the post laughing, sharing their own reactions to the photograph.
“It looks like it’s facing the opposite way of the rest of your face,” one person said.
Another added: “Oh Lord, they have done you dirty.”
One fan noted that it looked like someone had “edited out a turkey drumstick”.
“Did you have a dental appointment that day and was still numb from the injection?” asked a fourth person.
The Ghosts Christmas special will air on BBC One and BBC iPlayer on Christmas Day (25 December) at 7.25pm.