The actress's character Whitney Dean is present at the Branning clan's festive celebrations on December 25 as they invite their extended family to spend the day with them.
Viewers have been told to expect an explosive Christmas for the Brannings as Max's mysterious secret is finally revealed just as he is preparing to marry Tanya again.
McGarty told Take It Easy: "It was brilliant to watch Jo Joyner and Jake Wood do their scenes - they're just fantastic. I enjoyed the whole thing and there's a very exciting build-up. There are 18 people sitting at the dinner table, so you can imagine the drama!"
Asked how she felt when she first read the scripts, the actress replied: "Really shocked. It's fantastic though, and the outcome is going to shock the audience. There's going to be huge consequences to what happens on the day."
Whitney is currently in a relationship with Tyler Moon (Tony Discipline), but McGarty added that she isn't certain they belong together.
She said: "I'm not sure because she did kiss Joey. It's a bit awkward at the moment for her because she doesn't know what she wants. She wants to be happy and find 'the one', but I think she may still be looking."
EastEnders airs an hour-long episode on Christmas Day from 8.45pm on BBC One. Get the Inside Soap magazine on your iPhone or iPad