nly when absolutely badgered will Lenny Rush admit that the title role of The Reluctant Vampire, Val, was written for him: "Shane Allen, the executive producer, loved the books when he was young, and he was the exec on Am I Being Unreasonable?. So yes, the scripts were written with me in mind. I'm so happy they were so good. I'd have felt awful if they said 'We've written this for you' and …" he trails off, pulling a face of exquisite pained courtesy.
Rush likens the show to The Naked Gun and Airplane!, and it has the writing team behind Alan Partridge, so can conjure awkwardness out of thin air. But it also taps into a comic tradition that goes even further back: a delight in the ridiculous, people talking to bats and gargoyles, invisible women interjecting by surprise, a surrealism whose comedy comes from the audacity of acting it with such commitment. "In my opinion," says Rush, "sometimes things are labelled as a family show but they're aimed at kids. I really do get the feeling that The Reluctant Vampire is for everyone."
The Reluctant Vampire was written in the 80s by Eric Morecambe, in a surprising digression into children's fiction from the comedy giant. It's about the young scion of a bloodsucking household who doesn't have any skills: his teeth aren't pointy, he hates the taste of blood, and it transpires that he's not a vampire at all but a rogue human. In the hands of Neil and Rob Gibbons, it's like watching the essence of British comic sensibility – slapstick shot through with genuine pathos.
"You've got Tom Davis playing a vampire, this big character, with these big movements and this big voice, that can't not attract a younger audience," says Rush, "but the lines are genuinely funny." The show opens with Davis in full vampire regalia, ready to strike an elderly victim in her bed, only for Rush to derail it all by rushing in saying he needs a wee.
Still only 17, Rush has been a fixture on TV for many years, starting aged seven in CBeebies documentary series Our Family. Then, aged eight in Apple Tree House on CBeebies he played Eli, a young rascal on an estate of well-intentioned young rascals. If you have children this age, you'll feel like you've known him for ever, and if you are watching the latest season of Slow Horses, you'll know him as the foul-mouthed, ancient-beyond-his-years kid whose steel trap of a mind effortlessly vanquishes the spies.
But his breakthrough role was in Am I Being Unreasonable?, for which he won a Bafta at 14 for best male performance in a comedy – and he was nominated again for the same award this year. That role, Ollie, along with his magisterial Bafta acceptance speech, sealed his reputation as a guy who has hacked your mainframe, and could effortlessly make you laugh at any time.
You can see in Rush's performance as Val how much he's gorged on Mr Bean, Alan Partridge, Monty Python. "I probably watched some things I shouldn't have when I was younger," he says. Never mind the age-appropriateness, how he found the time is anyone's guess, since he has spent so long watching the British version of The Office that he knows it episode by episode, and has been working – as well as going to school – since he was seven. David Brent has always been his inspiration, and he delivers the kindest reading of that character I've ever heard. "You think: 'What an idiot.' Then you think: 'What a not very nice man.' Then you get to the end of series two, and there's a really big element of pity. He's just a man! Trying to find his way! He just wants to be part of the gang. It's actually a tragedy."
The opening episode of The Reluctant Vampire makes a lot of his stature, his parents measuring him and wondering if he'll ever be tall enough to be a vampire. Rush, who has a form of dwarfism known as spondyloepiphyseal dysplasia congenita, never went into acting for his condition to be a "talking point. I never thought I could be an inspiration for disabled people or young people." The impact of his visibility on the world around him came as a complete surprise. "When Am I Being Unreasonable? came out, I started to get messages from mums, saying: 'My little boy's got a disability or a form of dwarfism, he's struggling a bit at the moment at school. But he's seeing you on telly and it's giving him confidence.' It's so mad that I can be doing what I love and making people feel like that. I'm so pleased that that's how people have seen it."
Actually, though, the unvampiric vampire is a metaphor for belonging that is universal to teenagehood. "Going to school, trying to fit in, trying to impress, failing miserably. I think that's why it's so relatable. Even though it's about vampires. Or you could see it as someone trying to find their place in society: 'Am I doing this right? Am I good enough?' It shouldn't be so awkward, but it is."
He's not bringing a lot of his own experience of school to the travails of Val, incidentally – he never stopped attending, but was always off filming something and must have been artlessly cool. Which also made his parents very focused on his attendance. "My mum took school really seriously – not even necessarily, to be honest, the education side; she just found it very important for me to be seeing my mates. So I'd be on the PlayStation, and go: 'I'm working with Gary Oldman,' and they wouldn't even look up from the game. You can get swept away really easily, on set. You have assistant directors coming into your dressing room, asking what you want for lunch, it's quite insane. You have to have the right people around you."
After Am I Being Unreasonable?, Rush got two episodes of Doctor Who, even if he wasn't exactly a devotee. "I've not told Russell T Davies this, but I never watched Doctor Who. I knew what it was! I knew it was an honour to get asked. I knew it was a real moment in your career, especially to get cast young – I think Andrew Garfield was in it at a young age." He played Morris Gibbons, a 13-year-old genius and adviser in the science unit, a really smart move after the role of Ollie, in which he's also a genius but of a more cunning, domestic kind.
Am I Being Unreasonable? sometimes seemed to hang off his every expression – the plot of that show was absolutely wild, full of the worst parenting in the world, elaborately revealed secrets, dead pets. Rush's command of the mood was what tethered it to earth. "Ollie was the breakthrough part, and after that I saw myself as an actor. But when I'd look online, before and after, people would say 'He's a great comic actor' or 'He's a great young comic'. And I'm really thankful for that, but I never saw myself as a comedian or a comic actor. And as much as I love comedy, I don't want that to stick as my thing." His mum asked him recently whether he'd ever try standup. "It's a bit of an annoying thing about British culture, and quite sad in my opinion, that it's easy to get pigeonholed, and if I ever did try standup, it would take me further away from the Slow Horses kind of work."
His next role is Dobby in the Harry Potter series that's been beset by controversy – not just about whether actors can be in it without tacitly endorsing JK Rowling's views, but also bizarre racist death threats to cast members. Rush hasn't started yet, definitely hasn't had any death threats, and is tight-lipped, saying only: "It's pretty incredible – I feel so lucky, it's insane." It's casting against type, this house elf, because Dobby's not the effortless manipulator of all he surveys: his emotional top note is craven gratitude and anxious helpfulness. Maybe Val is a useful bridging role, because as a not-really-vampire, Rush ventures into new territories of panic and pitfall.
The Reluctant Vampire has palpable ensemble chemistry, between Rush, Sian Clifford as his mother, Davis as his dad, Asim Chaudhry as a Lurch-like character. Newcomer Gabriel Nagy is a great foil as his brother, a functioning, evil vampire. "I never felt like anyone was learning from me," he says. "I felt like I was learning from them. I never felt like number one on the call sheet, having to keep everyone in high spirits. I remember Tom telling me, it sounds obvious but it was so true and so clever – if you're doing a comedy, you want to make people laugh at home, so your goal in that scene is to make your co-star laugh. Try and get them to break." They're all pros, of course, so no sign of anyone corpsing; but like every classic comedy since TV began, you can feel that they're smiling on the inside.
The Reluctant Vampire is on BBC One on Sunday 4 October at 6.40pm.