Coleman will join Matt Smith in the sci-fi drama's 2012 Christmas special - read for all the details (that we know so far!) on the 25-year-old star's debut, in our full transcript of the press conference.
Steven Moffat (showrunner), what can you tell us about the character?
Moffat: "Absolutely nothing - you're going to have to wait and see! It's not the usual kind of story, it's a very, very different way for the Doctor to meet his new friend, so [I can tell you] absolutely nothing. Not even the name."
Jenna, how do you feel about being cast?
Coleman: "Beyond excited. Very excited - I want to get started already."
When do you start shooting?
Coleman: "I believe it's May time, [for] the Christmas special."
Moffat: "That'll be her first episode."
Are you a fan of the show and do you have a favourite companion from the past?
Coleman: "I'm a huge fan of the show. It's actually literally my grandma's favourite show, so she is the most excited person in the world! I think my favourite companion is probably... it's between Billie Piper and Karen Gillan, I think.
"I've got a lot of memories from the Billie Piper era, and obviously I've been watching quite a lot of the show recently as well."
And aside from Matt, who's your favourite Doctor?
"It's probably David Tennant, because it's the David Tennant and Billie Piper era that I remember the best. But it has to be Matt, of course!"
How did you win the companion role?
"Matt was in the audition with me and he made me feel like we were both in it together, which was lovely. And I just left the room on a massive high, with a feeling of, 'This would be great to do day-in, day-out, on set, together with Matt'.
"It just felt very spontaneous and fun, and you didn't know where it was going to go next, and we were both playing around. [Then] I went back in for another meeting after and I was excited to get back into the audition - I've never been so excited to go to an audition before!
"[The script] was called 'Men on Waves' and I knew Karen Gillan said hers was an anagram [of companion] 'Panic Moon' so I spent ages trying to figure it out, and I found out this morning that the anagram was 'Woman Seven'."
Where were you when you got the call?
Coleman: "I was in Marks and Spencer's, holding an avocado. I was debating what goes best in a salmon salad, and then I got a phone call from my agent and I was like, 'I can't carry on shopping now...'
"So I put my basket down and left Marks and Spencer's, but I couldn't tell anyone! So I just went for a bit of a walk and tried to digest [the news] - it's been quite surreal."
Steven, why did you decide to cast Jenna?
Moffat: "We did see a lot of brilliant actresses, because Andy Pryor is a brilliant casting director. There was a moment I remember, I think it was the second time she came in. She and Matt were together and they both looked frightened, doing a Doctor Who scene, it was brilliant.
"And then the fact that - which we're going to play a lot with - I think she's possibly the only person I've ever heard go faster than Matt. It was the first time we were going, 'My God, Matt's trying to keep up!' - it came to life as a partnership. We were so excited."
Coleman: "I think it's got to be the Daleks, but then I don't know what's going to happen, so I don't know who I'm going to meet or who I'm going to be up against. It's a mystery to me."
Are you prepared for the change in your life?
"I think so. I think it's maybe something that will happen in stages. I feel like maybe I've had a little taste of it, but I'm just taking everything as it comes and being excited about the job. Doing a good job is my focus, really."
Steven, was the character fully formed or did Jenna's casting inform the character?
Moffat: "Well, it's a bit of both. The scripts were written, but the way it kind of works is, it starts with you telling them who the character is, and when it works, they tell you who the character is.
"Just as a lot of the Eleventh Doctor is actually Matt, it's down to him. You have to let it develop that way, so it's both. It will develop and it will change."
Were you always thinking that the new companion would be a young female?
Moffat: "You don't think in those terms. You don't think, 'We now need to find a young girl' - I had an idea for a story and, as I say, it is quite different... so I can't really answer that without telling what's coming, which obviously I would never do!"
Steven, can you tell us more about how many episodes we can expect and when they'll be broadcast?
Moffat: "Yes, six of them will come out this year, including the Christmas special and then eight in the next year. Jenna will be appearing first of all in the Christmas special.
"It's too early [to say much about that] and I might change my mind. But I'm writing it right now and it does have a notional title."
And is eight episodes the full run for 2013?
Jenna, do you feel any pressure about taking on such a high-profile role?
Coleman: "I think, on the outside - and I try not to think of it from the outside - there's a lot of interest surrounding the show, which is great.
"But what I feel comfortable with is the meetings I had with Matt and how much fun we had so again that's what I'm focusing on and why I can't wait to get on set, read all the scripts and just start doing my job! So I'm trying not to look at those pressures and just be excited about the job in itself."
What's it been like living with the secret?
Coleman: "I'm so relieved for today. My flatmates have had no idea, I've had so many text messages this morning! My flatmates thought I was going to be going off to do this other job, and it's opened up this whole web of lies.
"I've realised that I'm actually quite a good liar, which is terrifying actually! A few members of my family knew, but it's a huge relief today to be able to tell my flatmates!"
Have you received any advice from past Doctor Who stars?
Coleman: "No, I haven't. But I am heading to Cardiff this week, so maybe I might bump into Karen at some point then. I've had a few conversations with Matt, who's been amazing.
"He text me this morning as well, wishing me good luck and, 'We're gonna have the best time ever', I think were his words! So he's been very supportive and excited."
Steven, can you reveal more about Amy and Rory's exit?
Moffat: "Yeah. Amy and Rory will leave in the fifth episode that goes out, and it will be a final encounter with the Weeping Angels, and not everybody gets out alive - and I mean it this time!"
And with their departure, does that mean we've seen the last of River Song (Alex Kingston) too?
Moffat: "No, not necessarily."
How long do you think Matt Smith will stay with the show?
Moffat: "Honestly we don't know. We take it a year at a time and that's the only sensible way you can go with it. It takes a huge amount of time, filming a series of Doctor Who and towards the end of that, you have to consider whether you're going to push on to another one.
"So the honest answer is, we don't know. We'll keep him as long as we can, that's the absolute truth. And he hasn't made up his mind yet."
Doctor Who will return to BBC One in the Autumn.