Digital Spy caught up with the quick-witted Mr Mack to chat about the show's return, how Not Going Out has evolved and his passion for studio-based sitcoms...
Is it easier or harder coming up with new ideas, going into a fifth series?
"A bit of both, really. On the one side, you've got to keep generating new ideas, but on the other side, you've got established characters. You know what they're doing, so it's alright.
"The downside to our sitcom is that it's not high concept. You don't go, 'This is about two blokes living on Mars, who are in some sort of gay relationship' - there's a lot going on there! Not Going Out is a pretty neutral sitcom - to quote the Seinfeld thing, it's not really about anything.
"When you're not about anything, it's hard to write about something. But the plus side is, you can put anything in the hole, so you can say 'This week, me and Tim are going to be a gay couple who go to Mars, in some fantasy sequence'."
That would be a great episode...
"It would be good!"
How close is your real relationship with Tim to your interaction on the show?
"It's not a million miles away. The difference is, if I say to Tim off-camera, 'You middle-class pr*ck', he laughs, and on screen, I make him grimace - that's the difference!
"He's very self-aware in reality that he is a little bit middle class and he jokes about it more than I do. But that's not much fun on a TV show, so he pretends he's a working class hero and denies his middle class namby-pambyness!
"The funny thing is, [the character] wasn't written for him - the character was originally Colin, who was a yuppie - that's how long ago we wrote it. He was a a city-dealing, Porsche-driving stockbroker.
"In the pilot - this is f**king amazing to say - Tim's got stubble and a skin-head! I said to him, 'Tim, I need you to be a bit tough, because you're going out with Catherine Tate - because she was in the pilot. Catherine played it like... she wouldn't take any s**t, and that's what Catherine's like anyway!
"So Tim had to up his game, otherwise you wouldn't have believed it. But then the cast changed and we thought, 'Let's write it around Tim, rather than Tim playing the part', so it changed from the pilot."
Is it easier writing now you have actors in mind?
"Oh yeah. I've realised that my strength is writing around people. I like finding a comedic voice. When we bring in new characters now, I very much look for who makes me laugh and then I write the joke [around them]. This is a show for comedians, more than comedy actors."
How much of your time does a series of Not Going Out take up, given all your other commitments?
"It's a long time. I would say, in the last seven years since the creation of this show, it's probably been 80% of my working life. That 80% is spread out, so it might be four years at 100% and then a year off - so it's one year of concentrating on stand-up, but then four years of only concentrating on this.
"It's most of every year usually. It's all about the writing - I spend most of my life writing this and occasionally I'm writing stand-up. When I'm on tour, it's easy because I'm not writing the show.
"Writing is the hard bit, hence comedians getting f**ked off when people nick their gags, cos that's the hard bit. It's not the doing bit - that's easy! So I spend most of my life going, 'F**k, I've got to write another series!"
A sixth series has already been confirmed - is it nice having that security given the show's turbulent past?
"It's nice knowing - to know before you're showing it, it's brilliant. I mean, the BBC could not be any more supportive of the show - the current regime. I mean, [BBC's controller of comedy commissioning] Cheryl Taylor and [BBC One controller] Danny Cohen have totally, unashamedly gone, 'We like the show, we're gonna do it' and I love that.
"One day, they might think it's s**t - they strike me as the kind of people who will go, 'You know what? We don't want it,' - they won't f**k us about. They're very to the point, and I like that. It's not like everything I've ever done for the BBC has been unanimously praised - I've had it both ways and I understand how it works."
Do you think the show's been a success because it has a broad appeal?
"I genuinely can't answer that question. We live in a world where, if this show was on at 3 o'clock in the morning on BBC Four, I'd be happy. When I tell a joke, I don't think about who's listening to it. I tell the jokes I think are funny.
"So I don't know about the broad appeal - I wouldn't have a clue. I know that I like it and I know that my mates like it, and that's as far as my brain can compute."
You're a champion of the studio-based sitcom. You must be pleased that Danny Cohen has called for more shows of that type?
"I didn't know he'd said that, but if he has said that, that's brilliant, because it's a genre that seen as a little bit old-school and not happening any more. The day we made the pilot for this show, there was a documentary called The Sitcom Is Dead - the then-head of ITV David Liddiment presented it and said, 'Sitcom is dead'.
"Victoria Wood was on it - she obviously represents studio-based sitcom. She said, 'Since The Office and The Royle Family, things have changed' - that was the day before we did our studio-based sitcom!
"The style of sitcom in studio is seen as dead because it's an unnatural way of speaking, and suddenly the natural way of speaking was in vogue. If you have a natural way of speaking [in your show], it doesn't suit jokes. People [in real life] don't tell jokes every five seconds.
"That's why we need the audience laughing, because if the audience don't laugh, the viewer says, 'People don't speak like this' - if the audience laughs, you know you're witnessing a live event. When people say, 'I don't like laughter on a TV show', I think, 'How do you cope when you're watching a stand-up gig live?' - it's the same thing!
"You're witnessing a live event and you're wrapped up in the world around it. But for some reason, at that time, escapist comedy was not in vogue. But it was great for me, because I genuinely had no choice - I couldn't do naturalistic to save my f**king life!"
Not Going Out returns to BBC One on Friday, April 13 at 9.30pm.