Hanssen's frightening ordeal begins when unhinged Nick Mooney (Sam Troughton) turns up at the hospital ready for a showdown, furious over the way his late father Ritchie was treated recently.
Outside in the car park, Hanssen enters Nick's work van in an attempt to talk to him. However, he quickly regrets this decision when Nick suddenly handcuffs him to a table and becomes threatening - with a nasty-looking spike gun in his possession…
Digital Spy recently chatted to Guy Henry, who plays Hanssen, to hear more about the episode.
Did you enjoy filming this big episode for Hanssen?
"Yes, definitely! What's fantastic from my point of view is that there is only so much looming in a corridor that you can do without boring yourself, the audience, or your fellow actors!
"One of the brilliant things about Holby is that all of the regular characters have their ups and downs in terms of story peaks and troughs, and it's now Hanssen's turn to have some really interesting storylines. I think we're going to see Hanssen as we haven't seen him before."
Can you tell us a bit more about this week's storyline?
"Well, the audience will know that a patient named Ritchie died in a previous episode. As a result of this, his son Nick is not happy at all - and he's not to be fobbed off by the usual response from the hospital. Now he's coming for revenge rather than an apology, so it's a case of whether Hanssen will see his point of view or not.
"We filmed the episode in quite hot July conditions, in a very small van in the Holby City car park. It was really intense, actually. You see Hanssen losing his cool somewhat, to say the least!"
Were the scenes a challenge to film?
"Yes they were, because the actor who's playing Nick - Sam Troughton - is a really nice man who I've known for a long time. We met when he was a barman at the Dirty Duck pub in Stratford-Upon-Avon, when I was at the Royal Shakespeare Company there in the early '90s.
"Doing these scenes with Sam in the van was really challenging, because he's a wonderful actor and I didn't want to let him down as someone who's known him for years. He came in to do this brilliant guest role, so I wanted it to be as good as it possibly could be. It was a real change and I was very excited to do it."
Prior to the hostage situation, Hanssen has been receiving threatening phone calls and emails. Why doesn't he alert the police beforehand?
"I assume Hanssen thinks he's so all-powerful and untouchable that nothing really serious is going to happen. But there's a moment where he receives one particular telephone call, and it's quite clear then that the man who is pursuing him is very close - right outside the hospital, in fact!
"Hanssen suddenly starts to feel a bit pale, and a situation which was initially just some nuisance phone calls and emails quickly becomes something else. I don't think Hanssen takes it seriously until suddenly he finds very unpleasant things happening to him…"
Do the producers give you much advance warning when you have such a big episode on the way?
"They do, yes. Rumours abound, and you are warned if you've got some good stuff coming up. It tends to be all or nothing at all on Holby. Sometimes I'll have a really easy week where I do a few scenes and then I'm off again, and then other times I'll be here from 7am to 7pm, which is quite a long day. I realise it's not mining or being a soldier in Afghanistan, but when they work you hard they really, really work you hard!"
Have you watched any of these scenes back yet?
"Not really - I'm a bit nervous about watching it. I did watch a couple of the takes at the time, but that's it. It's interesting that the scenes include some things that I haven't done before with the character. For example, Hanssen rarely shouts - I've only done one scene before where I shouted at Jac Naylor. But you do see Hanssen lose his cool in this episode, so it was quite intriguing to see how best I could play the scenes.
"It was very different for me. I watched some of it back, but I don't know how pleased I will be when I see it go out as an edited episode. I'm not always the best judge of my work - sometimes I hate what I watch when I watch things back, but fingers crossed I won't hate it too much!"
Which other episodes have been your favourites to film?
"The couple of episodes where Hanssen had to say goodbye to Sahira Shah. I found those very moving, partly because I got on so well with Laila Rouass off camera. It was really sad to see her character go.
"When I meet people who watch the programme, some people say Hanssen is horrible, others say that he's vulnerable - I suppose he's all sorts of things. I like it when there is a bit of complexity, and part of the great thing about his relationship with Sahira was that she was his protégée and he likes to encourage people who he thinks might have potential.
"I think that's also been quite clear in episodes where he's encouraged young people and teenagers. Hanssen is a strange fish, he's more passionate than people might think, and there's more going on underneath that calm surface."
Hanssen is still quite a mysterious character. Do you like that about him, or would you like the viewers to find out more?
"I think the viewers will be finding out some more things about him in these coming episodes. There's only so long you can carry on having a character as a total enigma who we really know nothing about. That's fascinating for a while, but I think it's been going on for quite a long time now, and I'm very glad that we're able to see different sides to the character.
"My father was an actor, and he did say to me quite a while ago, 'Come on Guy, we've seen that now - what else is going to happen?' I think we're starting to see some other things coming through now, and I'm delighted by that."
Hanssen's story with Sahira always caused a lot of discussion. What was your take on their relationship?
"It's a strange one, isn't it? Why does anybody care for and perhaps grow to love someone? I personally think that Hanssen did love Sahira, even if he didn't admit it to himself. I'm not certain it was a romantic love, but I think it was a fascination with her soul, her character and her brilliance as a doctor.
"Anyone with a spark, a zest for life and an enthusiasm for their work is someone that Hanssen loves, and Sahira had all of that and more. He would have liked them to be soulmates, but sadly that wasn't returned by Sahira."
Who are your best friends on set?
"This is going to sound slightly sickening, but I genuinely like them all! I'm proud that we are known in the business as a happy bunch of people to work with. We have so many guest actors who come in for just one or two episodes - I've done so many of those jobs myself where you come in, you don't know anyone, and you are so nervous.
"That's why I'm glad the Holby cast are known for being so welcoming and easy to work with, because otherwise it's miserable for the guest actors. We're not a horrible lot at all, so I like them all.
"We don't socialise much outside of work, because quite frankly we're here most of the time anyway talking to each other. But I love Paul Bradley, Hari Dhillon, James Anderson, Rosie Marcel and all the rest! They're all a fantastic bunch."
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Holby City airs on Tuesdays at 8pm on BBC One.