The 31-year-old said: "It felt very emotional to be doing my final episode. My mother is mortified, honestly she was at the front of campaign for me to stay and wasn’t happy when I said I was going to leave. But, when you’ve got to go, you’ve got to go.
"Of course, it’s very sad for me in many ways because everything is the last time. It’s the last read through, the last time I put on the bow tie and the last scene in the TARDIS."
But he is delighted with his final episode, which he thinks is going to be a "belter": "Steven [Moffat] has written a brilliant, adventurous, funny episode and I’m really thrilled with it. It feels wonderfully Christmassy.
The big Christmas special sees The Doctor rescuing Clara from a family dinner, before attempting to unravel a mysterious message that could have devastating effects for his own fate, as well as the universe's.
Writer Steven Moffat teased: "It’s his final battle and he’s been fighting it for a while. The Doctor is facing the joint challenge of a mysterious event in space that has summoned lots of aliens to one place and helping Clara cook Christmas dinner.
"There are also elements from every series of Matt’s Doctor, which will come to a head in this special. Things that we’ve laid down for years are going to be paid off."
However, Whovians all know that this is the episode where the next Doctor will emerge– even though Moffat admitted that Smith is technically the thirteenth Doctor, and the time-traveller has used up all twelve of his regenerations.
But Smith, who was the show's youngest ever Doctor, insists that after leaving the show, he will still be a Who fan.
The hunk, who is currently starring in a musical adaptation of American Psycho, said: "It might take me a couple of weeks to get my head around it. I think it was the same for David [Tennant] and I think it was the same for Karen [Gillan], when she watched Jenna come in. I don’t think it’s easy, but it’s not my show, it’s the fans show, so I’ll be a fan and then it will be my show."
Speaking of Jenna Coleman, who plays the Doctor's companion Clara, she will be gaining a whole new Doctor in the form of Peter Capaldi.
But while she is looking forward to the new, older Doctor joining the show– "It was kind of that moment ‘of course, makes sense’"– the stunning actress admitted that she was extremely emotional upon Matt's departure.
Meanwhile, Mistresses star Orla Brady has the daunting task of starring in the big Crimbo special.
The Irish actress explained the character of Tasha Lem, saying: "The Doctor goes to see Tasha as they are old friends. He needs her help and knows he will find it with her as although she is fearsomely powerful she is loyal to her friend."
But even though the Doctor Who Christmas special is becoming as classic a tradition as the Top of the Pops spesh and turkey leftovers, Orla isn't going to be watching.
She said: "My Mum and brothers are insisting on watching it on Christmas day, but I find it excruciating to watch things I'm in with other people so I will go for a pint with a friend."
The Time of the Doctor airs on Christmas Day at 7.30pm on BBC1.