The following post contains spoilers for the latest Doctor Who Christmas special titled “The Husbands of River Song.” But it also contains some great insight from River herself, Alex Kingston. So make sure you’ve watched the episode (go ahead, we’ll wait) and then come back and read what she has to say about Dr. Song’s first ride in the TARDIS alongside Peter Capaldi.

Everybody needs a secret weapon. For the most recent versions of the Doctor, it’s the sonic screwdriver. But for Doctor Who show-runner Steven Moffat, it’s the character River Song played by the fantastic Alex Kingston. The actress may have just taken her final ride in the TARDIS for the latest Christmas Special and spoke with Vanity Fair about how she became the most powerful weapon in Moffat’s arsenal and why she was grateful her character got such a loving goodbye.

Moffat wrote his first River episode before he was given the reins for the series and that two-parter—Silence in the Library/Forest of the Dead—saw Dr. Song interact memorably with David Tennant’s Tenth Doctor before (sort of) dying. Kingston thought she would never return and if this were most any other show, that would be it for Dr. Song. But this is Doctor Who, a time-traveling series that needs only shrug and mutter “wibbly-wobbly timey-wimey” in order to bring River back for more. And that’s exactly what we got during Matt Smith’s three seasons. Much, much more. In fact, it was Alex Kingston who Moffat called on to make sure his first episodes in the position of show-runner with a brand new Doctor (Smith) and companion (Karen Gillan) went off without a hitch.

“We shot that season out of sequence. There was a tremendous pressure on them to succeed but I think, ultimately, everybody assumed they would fail because David had created such a beloved character in the Doctor,” Kingston told Vanity Fair over the phone. “My episodes, four and five, were the very first thing they committed to film just so if they were a little unsure and that lack of surety showed it wouldn’t be revealed in the first episode.”

Moffat couldn’t resist calling on Kingston to help him, once again, smooth over a Who transition. “Obviously, we’ve just lost Clara so I didn’t want to go straight into a new companion,” Moffat said in reference to the departure of Jenna Coleman’s character. But Moffat admits he had another motivation: “I brought River Song back in because I thought there’s a possibility I’d never write it [Doctor Who] again so that’ll be my goodbye. I hadn’t written River for a couple of years and I’d always loved writing for her and I’d missed her.”

When asked how she felt about being Moffat’s secret weapon, Kingston joked, “Bring Alex in! I know. I should hope that I’m a little bit more than that!” But Kingston’s character is in the rare position of having been special to three versions of the Doctor; an honor only Elisabeth Sladen’s Sarah Jane Smith can also claim. But she’s also a character that, while hugely popular, reflected some problems of the Moffat/Smith era.

First of all there was the tangled web of River/Melody and her whole mythology. Moffat grew a little dependent, for awhile, on creating female characters who were puzzles for the Doctor to solve. But, more frustratingly, River never truly got her romantic due as the Doctor’s wife. David Tennant’s Doctor greeted her mostly with suspicion (and, ultimately, sympathy) having never met her before and Matt Smith’s treatment of her was often combative or exasperated. “It’s stupid. You embarrass me. Why do you have to be this?” he famously flung at her just before an uncomfortable wedding scene that many consider to be the pinnacle of a dysfunctional relationship.