Read on to find out what she had to say about the show, Harry, and the end of the series...
Obviously, the big question is what's going to happen with Harry and Ruth. We've heard it will be a bit complicated this year...
"Yeah, yeah it is. I think that's good. I rate them for that. All the writers are quite grown up about the relationship - they're not going to make it easy for Harry and Ruth. I have no idea what they're going to do, in all honesty. I know people always say that.
"They've told me and Peter [Firth] that they have about six or seven different ideas and they're not telling us any of them. At the moment I'm being courted quite vociferously by the home secretary, played by Simon Russell Beale. I get to do loads of scenes with him, so I'm having an absolute ball."
When you say Ruth is being courted by the home secretary - is that romantically?
"Me and Simon are playing it for that! No, he thinks she's been overlooked. It's really interesting. That's why I think Spooks is such a good series.
"When I read the script, I was really surprised that the writers had chosen to go in that direction. He gets her in a very soft place that I didn't think Ruth had, her pride about how good she is at her job, which she always plays down. She's always trying to just get on with her job and the home secretary works out how to get to her and say, 'You've been overlooked'."
Does that make Harry jealous?
"He doesn't know!"
Does it all start from the first episode?
"Yeah, it starts dripping in. They drip feed it from episode one. I don't know [how it develops] but I know it goes all the way through. It's not one of those stories that appears and then [stops]."
So will we see scenes of you and Simon on the Embankment?
"God, I hope so! I love walking along the Embankment being filmed. What is that? It's some weird, strange need in an actor. It's just the most exciting thing in the world to be in the centre of London with the London Eye behind you. It's so thrilling."
"I think it did last series, quite a lot. She was weighed down by that last series. I think she's changed significantly because of it. She's become a different person. She's lost a lot of her ease in the workplace, which I think is really interesting.
"When I think about how she arrived, she arrived tripping over, dropping files, swearing at lamps. She was very sweet, very naive, very comical. She's grown up in the series and not always in the best ways. The knocks that she's had, I think, have taken away some of her joy, which is really awful. Standing away and looking at her I really feel for her that she's lost some of that.
"I'm not sure if that [story] is going to come back in this series. She's certainly getting on with her job and her and Harry are trying to find a way of working together without bringing their personal feelings into the workplace too much. But it's sort of impossible for them because of what they mean to each other now. So it's as it's always been, in a sense, because it's there whether it's written or not.
"The writers are very keen on dropping little breadcrumbs through the series which I think is better than doing a big, full on, obvious storyline about it. I think it's more interesting."
We saw Ruth out in the field last series - will there be more of that?
"Not so far. I don't know why. Maybe they think, 'We let her out - she shot some guy' and they're not letting her out anymore! I do go out. I am going out a lot more.
"I'll tell you what's nice - it's very much more old-fashioned spying going on. It's a bit more of the old feel of that sort of '70s spying, for Ruth anyway. She's not going out and suddenly doing kung fu."
So secret conversations in St James' Park and that sort of thing?
"Lots of that going on. We've got a meeting coming up in an art gallery - I'm looking forward to that. Lots of that, lots of old school spying."
How does Ruth react to the new people on the Grid?
"The best thing is that because of the sort of time jump, Erin [Lara Pulver] has been working there for about eight weeks. She likes her, she thinks she's fun. They get on great. I think she just thinks she's brilliant at her job.
"It's very odd to have her in Harry's office - that bothers her. I don't know if you'll get to see that - I just think it bothers her. She's like that. 'She's in Harry's chair, that's weird'. Every time Ruth goes to see Harry's office she sees this young woman in his chair. But apart from that [they get on well]."
Often if two women are put together on television there will be some friction...
"Yeah, I'm pleased they never do that. It's great. I find that really tiresome when they do that. They've never done it in Spooks - Ruth's always adored her colleagues, irrespective of their sex. I don't think there's ever been anyone that she's disliked.
"They're all at the top of their game, all those young spies, so I think she has complete respect for all of them. They're all going out and risking their lives every day and she knows they are, because she's seen a lot of them come in and a lot of them die. So she's aware of what they're [doing] - I think she has complete respect for them."
"I don't know. I've been involved with it for nine years, on and off. I was talking to Hermione Norris [who used to play Ros] the other day about how she felt when she left, and we both agreed that the real thing that happens is that you form these relationships with cast and crew and this building. So much happens in your life over nearly a decade and it all gets tied up with not only the work, but with the people that you spent those ten years with and the fabric of the building. It's very odd.
"It's a very emotional thing to walk back on that Grid every year, wondering if you're going to come back next year. To be honest it's something that you always think about with Spooks because - it always sounds like a cliché when we say that we read the scripts and we don't know if we're there to the end, but you don't, and we have lost cast members. I was totally shocked when Ros went and we had no heads up on that. So you know that every year could be your last year.
"I have no idea how I'll feel - probably incredibly emotional. I've had a child while I've been here. Lots of things have happened."
The tenth and final season of Spooks begins on Sunday at 9pm on BBC One.