Even so, RTD’s script does manage to sketch out a sufficiently condensed, Cliff-Notes' version of the character so that Rose – along with all her strengths and flaws – still leaves a short, sweet, strong-enough impression. Right from the off, we're reminded of her selfish side, as she pointedly returns Mickey's goodbye kiss but not his "love you", before dashing off all smiles to her handsome new Doctor in the TARDIS. Once again, there’s a taste of Rose’s awe and wonder at discovering new horizons (“Oh, I’ll never get used to this. Never! Different ground beneath my feet! Different sky!”), as well as her quick-thinking under pressure, when she and the Cassandra-possessed Doctor have to make a quick ladder-escape that then leads to a vertical, body-swapping chase sequence (“Ladder! We’ve gotta get up!”; “Use the sonic screwdriver!”; “Cassandra, go back into me, the Doctor can open it!”). And we even get something of a catty, Doctor-ish mean-streak to Rose during her brief reunion with the flat 2D version of Cassandra at the start: she might take the moral high-ground on the value of things like change and human evolution, but she also isn’t above cracking a couple of snide sex jokes at Cassandra either, just for kicks and giggles...

But absence does make the heart grow fonder, doesn't it? And if Rose and Cassandra's only other shared scene together, back in The End of the World, already established how diametrically-opposed these characters were to each other – in their appearances, their class background, their wildly different definitions of humanity – the added wrinkle (sorry) of watching Rose literally become Cassandra in New Earth relies on the combined strength of both Billie’s performance and Russell’s writing having to "up the ante" even further, in a sense: playing up everything that’s vain and shallow (and very pointedly, un-Rose-like) about the character, while everything "Rose Tyler" more or less disappears. We still have to get a sense of who Rose is as a character in the story, even while Billie is busy playing a completely different character.

Indeed, from the Doctor’s perspective, there’s quite a few screamingly-obvious giveaways out-of-character moments involving Rose’s behavior that might suggest to him that she’s Not Herself; namely:

Showing up conspicuously late to Ward 26; long enough for the Doctor to assume she’s gotten lost, ask Sister Jatt to make inquiries at reception and eventually phone her up himself to find out where she’s got to.

Her posh voice and faux-Cockney accent. Yes, imitating colorful rhyming-slang over the phone might come across as playful or endearing, but doing it in person is just, um… a bit odd. And it doesn’t sound anything like South London Rose.

Her noticeably quieter demeanor when she finally rejoins the Doctor in Ward 26; mutely following him around and giving little to no explanation for why she went missing earlier. Also, her being uncharacteristically overconfident once the two of them have discovered the dark, sinister entrance to Intensive Care: she’s the first to stride inside, leading the way forward in a way that’s far more “jeopardy-friendly” than usual, even for someone like Rose.

Her sexed-up, seductive change of wardrobe: notably, returning to the Doctor’s side sans blue jacket, but with busty purple blouse and Wonderbra'd cleavage. Quite the fashion-statement to make around your New New Doctor!

Um, SNOGGING THE DOCTOR OUT OF THE BLUE?!? Even though initially, he kinda puts that down to his own irresistible charm…

Mincing, coquettish, sultry body-language: she’s suddenly daintily folding her arms, swinging her hips about like a seductress and generally carrying herself with an air that’s very smirky, flirty, and haughtier-than-thou.

Seriously advanced technical know-how: Rose, who’s usually out of her depth with off-world space stuff like cat-nuns and automatic disinfectants, unexpectedly knows the ins-and-outs of complex computer technology that’s meant to be billions of years ahead of her own time, and even takes to bossing the Doctor around about it. “Search the sub-frame”, indeed!

No real reaction to the Doctor’s ongoing “little shop” comment. To be fair, she didn’t really respond to it before either, down in the hospital reception, although she did have a look at where he was pointing at…

But while the Doctor does pick up on most of these suspicious signs almost immediately – because, as Cassandra herself states, he’s “dangerous and clever” and cleverly, dangerously keeps them to himself until a more decisive moment arises – the one detail he chooses to actually comment upon, aside from her change of voice; the singular defining trait which RTD’s script understands as being so key to Rose’s character that he carries it over as character-continuity from the first series and actually has the Doctor spell it out to Novice Hame (and us in the audience who weren’t paying attention!) under an angry, mistaken assumption that it’s the cat-nuns who are responsible; the single absolute quality which makes Rose more human than Cassandra, the self-proclaimed “Last Human”, will ever be, is...

Her compassion.

ROSE. WOULD. CARE.

Remember, New Earth seems to be asking us, this is Rose Tyler we’re talking about. The Rose Tyler we’ve gotten to know so well over the course of Series 1:

Who was disgusted by Cassandra’s racist talk of “pure humans” and “mongrels”, and appalled by the dodgy morality of allowing the Gelth to inhabit corpses.

Who, just like her mother, isn’t afraid of speaking her mind to anyone that rubs her up the wrong way.

Who often appears profoundly uncomfortable with the notion of privilege founded on distinctions of class or wealth, and has no time for the rituals of hierarchy.

Who instinctively took exception to people like Henry Van Statten or the Forest of Cheem woman Jabe, that reveled in their own fantasies of social status and discussed her as though she wasn’t there.

Who was visibly shocked when told she had to give lowly Platform One plumber Raffalo permission to speak, was similarly shaken by servant-girl Gwyneth’s low expectations in the employ of Mr. Sneed; and lifted downtrodden street-urchin Nancy’s desolate spirits with a hopeful glimpse of a post-Blitz future.

Whose empathetic nature allowed her to step in and comfort those in distress, like Harriet Jones, or even put in a gentle plea for the Doctor to help a stricken Cassandra, in spite of all her murder and deception beforehand, once Cassandra found herself on the wrong end of her own sabotage.

Whose gentle, misplaced sympathy for a lonely battle-scarred Dalek nevertheless reawakened the virtues of pity, empathy and humanity inside an equally lonely, battle-scarred Doctor who was spiraling from the staggering trauma and losses he suffered in the Time War.

And who, despite the fact she was personally feeling lost and alone and utterly defeated after the Doctor’s new change of face at Christmas, still made the brave decision to step up before the Sycorax and speak on behalf of the whole of Planet Earth in his absence. Cos “someone’s gotta be the Doctor...”

Once she takes over Rose’s body in New Earth, Cassandra shows little regard for Chip’s possible bereavement at losing the flat, skin-form Cassandra he used to care for (“The brain-meat expired! My old mistress is gone…” “But safe and sound in here.”) and even less consideration for the fate of her host, whose body she has no qualms about objectifying (“It’s like living inside a bouncy castle!”) and whose own suppressed self, we later learn, is slowly being crushed to death inside her own head (“But what of the Rose-child’s mind?” “Oh… tucked away.”).

In Intensive Care, Cassandra’s gut-reaction to witnessing the horrendous, lifelong suffering of the plague-carriers is one of complete apathy, callousness; even revulsion (“That’s disgusting. What’s wrong with him?”). She asks questions like Rose might do (“How many patients are there?”), but her curiosity is more one of morbid fascination, solely predicated on how she can potentially exploit these horrors to her own benefit (“Just to confirm… none of the humans in the city actually know about this?”). And of course, her only concern is for herself (“What about us? Are we safe?”).

That’s the difference the Doctor sees the clearest, and hears the loudest:

Rose would care.