Let's indulge in a spot of time travel. Setting our own Tardis's coordinates for Cardiff, Tuesday March 28, 2006, we arrive at Wales' impressive Millennium Centre to find a preview screening room packed with journalists.

They're enthusiastically applauding the end credits to the latest episode of Doctor Who.

Entitled New Earth, the adventure - on BBC One tomorrow - sees the Doctor (David Tennant) and Rose (Billie Piper) stumble upon a plague farm in the far future run by evil cat-like nuns people, and encounter an old enemy, Cassandra (Zoe Wanamaker).

In a typically sci-fi turn of events, the vain last human alive takes control of Rose's body, allowing Billie the opportunity to vamp it up as a "posh bad girl".

"I love it," laughs the 23-year-old actress, when asked how she enjoyed playing the possessed Rose. "Love the posh bad girl."

Alongside co-star David Tennant, she's facing down a tidal wave of questions at the post-screening press conference.

It's a good-humoured event, the actors frequently collapsing into fits of laughter and often finishing each other's sentences.

"It was really fun," continues Billie, "but I was a bit nervous because it was quite a departure from how I normally play Rose.

"Plus, it was the first time in Doctor Who I've played quite a lot of comedy, so I was a little bit scared. But I think it works OK. I like it. It's fun."

And then, thoughtfully: "I'd like to play many more posh bad girls."

With David chuckling away beside her, it's obvious there's chemistry between the two.

"Fortunately, we get along," she says when asked what it's like to work together. And this is despite the fact he's just made a crack about her flatulence being the biggest problem on set. "It really is!" she confirms, with a laugh.

"We all get along on the show and we need to, really, because it's long months and it's quite intense. We're shooting 13-hour days, so you're thrown together all the time."

Then, turning to David, she says: "I feel like I've made a friend for life. I see you more than I see my family or my boyfriend."

It's an arrangement that seems to suit Billie's mother quite well. She, too, has taken rather a shine to David.

"My mum's on the phone all the time," chuckles the actress, "and that definitely started when she saw him in Casanova last year. Now she's just like, 'Are you sat next to him right now?"'

In this episode, the Doctor and Rose's relationship takes another twist, as the two lock lips.

It's the second time the plucky companion has played tonsil tennis with the Time Lord, following her clinch with Christopher Eccleston's incarnation last year.

"Kissing David is different from Chris," she says. "Both are great. I had a longer kiss with David, though, which I really enjoyed. It was quite sexy. He's got quite nice lips."

Despite the romantic encounter, she's not keen for things to progress further.

"I think the viewers want it to happen," she muses, "but at the same time they don't. It's one of those weird things, a bit like Ross and Rachel in Friends. You want them to get it back on, but then you don't so you have the anticipation."

In the meantime, she's more concerned with chiding her co-star about his obsessive Doctor Who knowledge.

"Ask him anything about the show!" she giggles. "Go on! I love this game. He's so good at it."

It's well-known that before taking on the mantle of the Doctor, David was a hardcore fan of the series.

When challenged now to name the constellation in which the Doctor's home planet Gallifrey, is situated, he mutters almost under his breath, "Kasterborous".

On the other hand, Billie's knowledge of Who lore is decidedly sketchy.

"I'm still a bit crap, let's be honest," she says, "so please don't ask me anything, because I'll just panic and start sweating. I can't cope with it.

"Rose doesn't know what's come before, though, so it works quite nicely. But, yeah, I have learned bits and bobs. The fans have taught me quite a lot.

"What's funny is that I didn't really understand sci-fi before I started Doctor Who. I just didn't get it. Now, I'm loving it more and more. So I think the fact I understand the concept is a good place to be. Then I'll start exploring the past and what's gone before."

Despite the obvious fun she's having on set, Billie admits the strenuous production schedule leaves her exhausted come the weekend.

"I just sit on the sofa for two days and don't really do that much," she reveals. "I don't even want to talk to anyone. I like to sit in my flat and be completely selfish, watch TV, eat far too much food and drink some beers."

Nevertheless, she feels happy with where her career has taken her.

"It's an exciting time for me to be alive," she beams, "and to do something I've wanted to do since I've been a kid. That means so much to me.

"Being here tonight and watching the episode, I just can't quite believe that it's all really happened. I'm really loving it and want to continue to do it for years. But all of those things are just a bonus, really. The fact that I'm working every day as an actress makes me really happy."

Most importantly, for fans who've grown to love the character of Rose, Billie confirms she's got no plans to leave the series.

"Yes," she says chuckling, "I'm sticking with the Who."

* Doctor Who is on BBC One from Saturday April 15