Directly after this episode aired there was a special interactive episode on BBCi, Attack of the Graske. In which you (the viewer) had to help the Doctor stop an invasion by alien bodysnatchers.

"The Christmas Invasion" took 22 days to film, with 28 cameras and sound tapes.

"The Christmas Invasion" was actually filmed in the middle of July.

500 supporting artists were employed for this episode.

Harriet Jones's Christmas Speech was the first thing shot.

The inside of the spaceship was shot in Clearwell Caves.
The outside was shot on a slag heap in Barry Island.

David Tennant is the first actor under the age of 35 to play the Doctor since Peter Davison left the role in 1984.

As the Sycorax ship descends on London, scaffolding can be seen around Big Ben. This is a reference to the events of "Aliens of London" in which an alien spacecraft struck the clock tower.

Before being cast as the Doctor, David Tennant appeared in several Big Finish Doctor Who audio dramas, including "Colditz" in 2001 and "Sympathy for the Devil" in 2003.

Rose: Both working.
Jackie Tyler: What do you mean, both?
Rose: Well, he's got two hearts.
Jackie Tyler: Oh, don't be stupid.
Rose: He has.
Jackie Tyler: He has.Anything else he's got two of?

Jackie Tyler: I'm gonna get killed by a Christmas tree!

The Doctor: My head, argh, I'm having a neuron implosion. I need...
Jackie Tyler: What do you need?
The Doctor: I need...
Jackie Tyler: Just say! Tell me, tell me.
The Doctor: I need..
Jackie Tyler: Painkillers? Aspirin?
The Doctor: I need...
Jackie Tyler: Codeine, paracetamol? Oh, I don't know, Pepto-Bismol?
The Doctor: I need...
Jackie Tyler: Liquid paraffin? Vitamin C, Vitamin D, Vitamin E!
The Doctor: I need...
Jackie Tyler: Is it food? Something simple. Bowl of soup? Nice bowl of soup? Soup and a sandwich? Oh, soup and a little ham sandwich?
The Doctor: I need you to shut up.
Jackie Tyler: Oh, he hasn't changed that much, has he!?!

The Doctor: We haven't got much time. If there's pilot fish then... why is there an apple in my dressing gown?
Jackie Tyler: Oh, that's Howard's, sorry.
The Doctor: He keeps apples in his dressing gown?
Jackie Tyler: He gets hungry...
The Doctor: What, he gets hungry in his sleep?
Jackie Tyler: Sometimes.

Harriet Jones: Harriet Jones, Prime Minister.
Danny Llewellyn: Well, yes, I know who you are.

Daniel Llewellyn: You seem to be talking about alien lifeforms as a matter of fact.
Harriet Jones: There's an Act of Parliament banning my autobiography.

Danny Llewellyn: But if they're not from the surface, then they might not be from Mars itself. Maybe they're not actual Martians.
Major Blake: Of course not. Martians look completely different.

Major Blake: The President is insisting that he takes control of the situation.
Harriet Jones: You can tell the President, and please use these exact words; he's not my boss and he's certainly not turning this into a war.

Mickey Smith: Tea. Like we're having a picnic while the world comes to an end. It's very British.

Alex Klein: (translating) "The yellow girl, she has the clever blue box. Therefore she speaks for your planet."
Harriet Jones: But, she can't!
Rose: Yeah, I can.
Mickey Smith: Don't you dare.
Rose: Someone's got to be the Doctor.
Harriet Jones: They'll kill you!
Rose: It never stopped him. I, um...I address the Sycorax... according to... article fifteen of the Shadow Proclamation. I command you to leave this world with all the authority of... the Slitheen Parliament of Raxicoricofallapatorius and, um... the Gelth Confederacy... as, uh, sanctioned... by the Mighty Jagrafess and... ooh, the Daleks! Now, leave this planet in peace!

The Doctor: Now, first things first. Be honest. How do I look?
Rose: Umm... different.
The Doctor: Good different or bad different?
Rose: Just... different.
The Doctor: Am I...ginger?
Rose: No, you're just sort of brown.
The Doctor: (disappointed) Oh, I wanted to be ginger. I've never been ginger! And you, Rose Tyler, fat lot of good you were. You gave up on me. Oh, that's rude. Is that the sort of man I am now? Am I rude? Rude and not ginger.

The Doctor: See, that's the thing, I'm the Doctor, but beyond that, I... I just don't know. I literally do not know who I am. So I'm testing. Am I funny? Am I sarcastic? Sexy? (winks at Rose) A right old misery? Life and soul? Right-handed, left-handed, a gambler, a fighter, a coward, a traitor, a liar, a nervous wreck? I mean, judging by the evidence, I've certainly got a gob! And how am I gonna react when I see this? A great big threatening button. A great big threatening button which must not be pressed under any circumstances. Am I right?

The Doctor: Look at these people. These human beings. Consider their potential. From the day they arrive on the planet and, blinking, step into the sun, there is more to see than can ever be seen, more to do than... no, hold on. Sorry, that's The Lion King. But, the point still stands. Leave them alone!

Sycorax Leader: You stand as this world's champion?
The Doctor: Thank you. I have no idea who I am but you've just summed me up.

The Doctor: You cut my hand off! And now I know what sort of man I am. I'm lucky 'cause quite by chance, I'm still within the first 15 hours of my regeneration cycle. Which means I've got just enough residual cellular energy to do this.
(The Doctor's hand grows back)
Sycorax Leader: (astonished) Witchcraft!
The Doctor: Time Lord.

The Doctor: This new hand... it's a fightin' hand!

The Doctor: Not bad for a man in his jim-jams. Very Arthur Dent. Now, there was a nice man.

The Doctor: (quietly to himself; after killing the Sycorax leader) No second chances. I'm that sort of man.

The Doctor: (to the Sycorax) By the ancient rites of combat, I forbid you to scavenge here for the rest of time. And when you go back to the stars and tell others of this planet, when you tell them of its riches, its people, its potential, when you talk of the Earth, then make sure you tell them this. It is defended.

The Doctor: Don't challenge me, Harriet Jones! 'Cause I'm a completely new man! I could bring down your government with a single word.
Harriet Jones: You're the most remarkable man I've ever met, but I don't think you're quite capable of that.
The Doctor: No, you're right. Not a single word. Just six.
Harriet Jones: I don't think so.
The Doctor: Six words.
Harriet Jones: Stop it.
The Doctor: Six. (He walks over to Alex) Don't you think she looks tired?

Jackie Tyler: Well, I reckon you'r mad. The pair of you. Its like youi go looking for trouble.
The Doctor: Trouble's just the bits in between! It's all waiting out there, Jackie. And it's brand new to me. All those planets, creatures, and horizons, I haven't seen them yet. Not with these eyes. And it is gonna be... fantastic!

The Doctor: Not bad for a man in his jim-jams, very Arthur Dent...now there was a nice man.

This is a reference to the character “Arthur Dent” from the novel and the BBC TV movie The Hitchhiker’s Guide to the Galaxy. Arthur Dent is whisked away on a wide ranging off-world trip while dressed only in his pajama’s.