Tenth Doctor, Fourteenth Doctor, and Metacrisis Doctor Duplicate actor David Tennant does a podcast called, imaginatively, David Tennant Does a Podcast With, and this week he picked up an extremely crowd-pleasing guest in the form of his old boss, Russell T Davies.If you were hoping for Frost/Nixon, youd be disappointed. This was a cosy chat between old friends, chatting about Davies childhood, how he got into a career in television, the highlights he enjoyed along the way, and, not surprisingly, a few Doctor Who memories. RTDs earliest Who memory was the regeneration of William Hartnells First Doctor into Patrick Troughtons Second.Davies recalls, The thing I remember is the switches on the TARDIS were moving on their own, which was so frightening.That fear, it turns out, would become a driving force for Davies. Later on in the conversation, Tennant asks why Doctor Who is the show that inspired not just Davies, but so many other people to work in the television industry. Davies answer comes quickly.At that age, its obviously the terror, the strongest emotion you will feel when watching television. That and laughter, Davies answers, later adding, Most television kind of makes you smile and just burbles along and might make you excited if theres a chase.Those two ingredients, scaring you, and making you laugh, are the things Davies believes make for the most powerful television.You feel it more than you would feel anything else, he tells Tennant. We all loved the Famous Five or Grange Hill or stuff like that, but you wouldnt quite feel it in the way you feel terror and you feel laughter. Its just on that size of things. Its big, and when its frightening its terrifying.When you put it like that, its perhaps not so hard to see why Doctor Who has had the impact that it has. From the start, the show has straddled the line between the genuinely terrifying and the terrifyingly hilarious.Monsters to Make You Scream with Laughter?If youre a lifelong fan of Doctor Who, somebody who, as former showrunner Steven Moffat says, is annoyed that its a kids show rather than the serious science fiction drama it was when you were eight, then the chances are you have at least one core, primordial moment of fear that came from watching an episode of Doctor Who.For me, its watching the shop mannequins come alive in Spearhead from Space (from a rerun in the nineties, Im not that old). For younger fans it might be the moment where Richard Wilsons face morphed into a gas mask in The Empty Child, or the entirety of Blink (although both my kids insist thats not scary at all and dont know what I make such a big deal about). For other elder Millennials and Gen-Xers, it might be the mutant haemovores in The Curse of Fenric, or for older fans, the titular Robots of Death.But just as often the Doctor Who baddies are just plain laughable. So many supposedly evil geniuses are little more than foils for the likes of Tom Baker, Jon Pertwee or Patrick Troughton to make a fool of. The Slitheen are very deliberately a joke from beginning to end, from their farting gas exchange on their skin suits, to their home planet Raxacoricofallapatorius (Thought Id spelt that correctly first go, I hadnt. Im more upset about than I care to admit). Even supposedly top-tier (lets be honest, secondary-tier) villains like the Sontarans are basically potatoes on parade.Join our mailing listGet the best of Den of Geek delivered right to your inbox!And thats aside from the Doctor themselves, who frequently seems like they have wandered into a horror movie from the wrong genre.You might have noticed that weve not mentioned the top baddies so far. You know the ones. And there is a reason for that because when Doctor Who is really on fire, when it is properly at work burning itself into the fabric of young minds, it is doing the terrifying and the funny simultaneously.The most deadly and iconic villain in the entire series is a pepper pot armed with an egg whisk and a toilet plunger, with a voice like a robotic toddler throwing a tantrum. And it cant even get upstairs no, get out of here with your Dalek and Remembrance of the Daleks retcons when we first meet the Daleks in 1963 they can be immobilised by putting a thin cloak between their base and the metallic floor they scoot around on.Yet they are still the biggest bad in a universe of big bads. Whenever those round heads with their little wobbly eyestalks and the lights that flash when they talk (ridiculous) appear, your breath catches for just a moment. You know the stakes have just got higher.The Daleks are the supreme beings of the middle bit of the Venn Diagram between hilarious and scary, but they are not its only occupant. Shop window mannequins. Whos scared of shop window mannequins? Or a monster that cant move when youre looking at it? How rubbish is that?Sylvester McCoys Seventh Doctor literally fought the liquorice all-sorts mascot, Bertie Basset, and it was one the most gruesome and terrifying bits of the shows history.There is a reason why Doctor Whos most memorable, and most memorably terrifying villains are also sort of ridiculous. Its because the two big feelings Russell T Davies says TV is good for are ultimately one and the same thing.Not Ha Ha Funny But Ha Ha TerrifyingIt isnt a coincidence that Steven Moffat, the writer known for providing some of the greatest scares in the shows recent history started as a sitcom writer. What Doctor Who does well is show us the thin line between comedy and horror.The truth is that scientists are likely to be arguing for a long time to come over why we evolved laughter and what function it serves, but there is no denying that one way or another it is linked to fear. Hyenas laugh when they feel threatened or under attack. If youre a parent, the odds are you have tried to make your baby laugh by convincing them theyve been abandoned (by hiding behind your hands) or maybe threatening to eat them.And returning to stories, once you remove all emotion (which sounds like a totally okay and cool thing to do), a ghost story and a good joke have ultimately the same structure set up, establish a pattern, twist, and then a reveal that pulls the rug from under you.So Russell T Davies, who in his conversation with David Tennant describes himself as the last terrestrial television viewer might think that TV is best at provoking two big feelings, but it turns out that in the end, it is just one feeling.It does it very well though.Doctor Who returns to BBC One, BBC iPlayer and Disney+ on Saturday April 12.