IF a willingness to embrace outlandish hairdos and challenging costumes is any measure of worth in an actor, David Tennant is up there with the best – as he has shown in the past, and as his next few screen outings will further demonstrate.

The 46-year-old is currently in South Africa where he's filming TV series Good Omens, an adaptation by Neil Gaiman of his own 1990 fantasy novel, co-authored with Terry Pratchett. Tennant plays a demon, a part which requires the sort of hairstyle last seen in a 1980s Goth band: long, swept-back and dyed an unfortunate shade of rust. It's not unlike the copper-coloured hair extensions Tennant wore to play Richard II on stage in 2013, though here the look is completed by round sunglasses, a bolero-style jacket, tight jeans and a skinny tie. Worn with a T-shirt.

At the other end of the scale, Tennant recently completed filming on historical drama Mary Queen Of Scots in which he plays Protestant firebrand John Knox, generally portrayed as heavily robed and sporting a two-foot long beard with closely-cropped hair. Cue another long session in the makeup department.

And before all that comes British rom-com You, Me And Him, which has its European premiere next Sunday at the Glasgow Film Festival and which sees Tennant unleash his inner hipster as a man bun-sporting roué with a penchant for muscle tops, ill-advised rock star jewellery, leather trousers and extravagantly-sculpted facial hair.

“It was nearly all my beard,” Tennant laughs when I ask about this latest tonsorial makeover. “It was 90 per cent my beard, with a sort of promontory at the front which was added, little tufts of yak hair just to give it that little bit extra. I had it long enough to get quite a full carpet round my chin, but not quite long enough to have the little sculpted extra that gave it the full hipster.”

Written and directed by Daisy Aitkens and produced by Tennant's actress wife Georgia Tennant, You, Me And Him tells the story of same-sex couple Olivia and Alex, whose lives are thrown into disarray when they both end up pregnant – Olivia thanks to an IVF procedure (planned), Alex thanks to a fling with divorced neighbour John after a very drunken party (unplanned, obviously).

Olivia is played by comic actress Lucy Punch (Motherland) and Alex by Games Of Thrones's Faye Marsay, most recently seen as Katya Godman in Russian crime thriller McMafia. Also on board are Tennant's Broadchurch and W1A co-star Sarah Parish, Nina Sosanya (another W1A alumni), Gemma Jones, David Warner, Sally Phillips and Simon Bird. Oh, and there's a cameo for another former Doctor Who, Peter Davison, who just happens to be Tennant's father-in-law. All in all, it's quite a line-up.

Tennant, of course, plays John, the ageing Scottish hipster next door whose pride and joy is a signed football (team unspecified) and who runs a website called Mannism devoted to all things alpha male. John's world isn't one Tennant says he has much experience of, “but it's quite fertile ground for characterisation, especially as he's a little bit too old for it. And that's very much the point really. He's a man living a youth that he somehow felt he missed out on – unhappily married for many years and then sort of released into the white heat of a midlife crisis. So when he gets involved in the lives of the women next door he wades in with both feet and becomes – well, he believes indispensable. They would say probably incurable.”

Tennant tackles the role with glee, as you'd expect. Punch and Marsay are endlessly watchable and there are some whip-smart lines as well as a few glorious set-pieces, such as the cringe-inducing antenatal class overseen by Sally Phillips's Australian birth guru or the garden party attended by Olivia's butch, undiplomatic and entirely humourless childhood friend Biggles.

But behind the laughs, You, Me And Him has some serious points to make about modern family units and what is considered normal in the late 2010s. As Tennant points out, beyond the fact that both Olivia and Alex can be pregnant at the same time, their being a same-sex couple isn't really the point.

“It's depressingly rare, I think, that a gay couple should be at the centre of a story and their gayness is not the issue which they're being asked to deal with,” he says. “All the families we see are very different and yet all equally important and relevant and valid. I think all the characters that Daisy creates are very recognisable. It feels like a modern story and it feels like it's very much a story of the nation we live in right now in terms of its variety.”

For Tennant, another part of the appeal was the chance to work again with old friends Sarah Parish and Nina Sosanya.

“Sarah and Nina are probably the two people I've worked with on the highest variety of productions,” he says. “I think I've worked with Nina about 10 times and Sarah six or seven, and from a personal point of view I love that. I love the shorthand it affords you and I like them both hugely as people and performers.”

That said, it wasn't always certain that Tennant would even be offered a role in the film. The way he tells it, he wasn't first choice.

“They tried various people for this part of John and I was sort of watching them approaching various actors who weren't available or couldn't do it for whatever reason. And I just sort of sat there very quietly wondering if I was ever going to get a shot and they obviously realised that I might still be available, and I got brought on board. But I was far from first choice. But I'm very happy about that. I feel like I'm big enough to rise above it.”

Good Omens, a big budget production to be screened on Amazon Prime, is a different story altogether. When the first teaser images of Tennant and co-star Michael Sheen in costume were released last year, there was a feeling abroad that this was about as close to a Celtic acting dream team as you could get – Tennant, a former Doctor Who and star of Broadchurch and Harry Potter, alongside chameleon-like Welshman Sheen, who has played everyone from Brian Clough to Tony Blair and Kenneth Williams. But although both men knew each other well, they had only acted together once before: 15 years ago, on Stephen Fry's Evelyn Waugh adaptation Bright Young Things. And even then they never actually shared screen time.

“Although we spent quite a lot of time on set and in prep and socialising a little bit, we didn't have a single scene together. So I've known Michael all that time and never really had a chance to properly act with him until now. So when Good Omens came up, and I knew Michael was involved, that did feel like I could finally tick a box I'd been hoping to tick since I first met him.”

And how was the box-ticking experience?

“It's always a gamble working with someone, even someone like Michael who I knew relatively well,” Tennant admits. “But at the same time it can be hugely exciting to discover a new working relationship and that's mercifully been the case with Good Omens. Just about every scene I have it's Michael and I staring at each other and that could have been a grim experience over the six months of the shoot if he hadn't been such a joy to be around.”

In fact, Tennant is feeling “rather excited” about Good Omens. He, Sheen and the rest of the crew recently viewed a “sizzle reel”, a rough trailer put together by the show's Portree-born director Douglas Mackinnon. And he liked what he saw.

“It's quite hard tonally to get a grip on what Good Omens is, because it's this very unique world that comes from Terry and Neil's novel and from the scripts, which Neil has adapted pretty faithfully from that novel,” he says. “I think it's quite unlike anything I've ever been in before and possibly anything many people have seen before. It's like a sort of fairy tale with a kind of very real world setting. It's a farce and it's also deeply serious, It's all things at once and not quite any one of them. If the rest of the show turns out like this early trailer that we've all seen I think it's going to be quite special.”

Beyond the show itself, the involvement of Amazon also makes Good Omens feel a little out of the ordinary. For a start there's the amount of cash available to the show – “We've clearly got a bigger budget than any BBC show I've ever worked on before,” says Tennant – and then there's the palpable sense of ambition and expectation coursing through the production.

“That whole Amazon-Netflix model seems to be the future and there's a sense that you're working with a company that people are watching very keenly at the moment to see what's going to happen next. From an acting point of view you just turn up and you do your job like you do on anything else. That's the same on Good Omens as it was on a low budget British film like You, Me And Him. The nuts and bolts of it are the same every day. But it's when you take a step back you see the ambition of something like Good Omens and the resources that we've got to play with. It's very exciting to be able to be part of that.”

Good Omens is scheduled for release next year. Before then, a very different role for Tennant: John Knox in historical drama Mary Queen Of Scots.

Directed by theatre-maker Josie Rourke, artistic director of London's Donmar Warehouse's, it stars Saoirse Ronan as the titular queen opposite 2018 Oscar nominee Margot Robbie as Elizabeth I. Also in the cast are Scots Jack Lowden and Martin Compston.

Hairstyles and costumes aside, Mary Queen Of Scots provides Tennant with the sort of heavyweight role he has often undertaken on stage during his long association with the Royal Shakespeare Company. In fact you could almost see Knox as a Shakespearean villain, a flawed character as convinced of his own moral rectitude as he is sure of others' sins. Did Tennant find anything at all to like in the man? Not much, he says.

“I don't think the screenplay regards him particularly kindly either, to be entirely honest. But as an actor I guess your job is to find the reasons for why someone does the things they do, so that's what I set out to do. John Knox comes from a different time and I wouldn't say that on a personal level I can identify with some of his beliefs, certainly the more misogynistic ones which are particularly focussed on in this portrayal of him.”

Still, he adds: “He was clearly a man who felt motivated to do what he believed was the right thing and that was all that mattered, and God's will should be done. That's something you can certainly key in to as an actor. You can get behind the passion of that and the conviction of that.”

Knox may come from a different time, but the religious schism in which he played such a prominent role is still present in Scottish society, as Tennant well knows – a son of the manse himself, in a much-reported 2011 episode of BBC genealogy programme Who Do You Think You Are? he journeyed into his family's strong links with Northern Irish protestantism.

Did that family history inform his performance at all? “My father was a minister in the Church of Scotland, so that's always been something that I've been very aware of,” he says. “My father was about as far from John Knox as I imagine it would be possible to be. He was a great deal more progressive and liberal and outward reaching than John Knox appears to have been. But the fabric of all that is something I grew up with, even before I'd looked into my Northern Irish ancestry, where of course that was written very vividly, even more so than I ever knew. It's still something that carries on. It's not something that I identify with, but it's something I'm very familiar with, certainly.”

But however many different hairstyles, costumes and characters David Tennant adopts, there's always one role that any and all interviewers will want to return to: Doctor Who, the space-travelling, police box-dwelling, Dalek-confounding Time Lord so beloved by generations of Britons. Tennant played the Doctor for five years between 2005 and 2010 and – though it's a subjective opinion which depends very much on your age – to many he's been the best of the 21st century doctors so far. So what I want to know of him is this: when it's time for the inevitable regeneration and for a new man (or woman!) to pick up the mantle, how hard a role is it to come out from under?

“When I went into it I knew that this would stick to me for the rest of my natural life in all likelihood,” he says. “You're not unaware of that. I don't think you quite know what that means until you've lived it, of course. You can understand these things intellectually without knowing what you're going to feel like emotionally when people are pointing at you and whispering in Sainsbury's. But I wouldn't for a second suggest it's something I would want to get out from under. But equally I think I'm wise enough to understand that I can't really. You have to just enjoy what that brings.”

And he does. At the end of the day, he says, very few people hate Doctor Who. Nobody is angry about the fact that it exists. It is, ultimately, a “benevolent thing” to be part of. “So I feel very blessed.”

David Tennant will be attending the European premiere of his new film You, Me and Him on February 25 at Glasgow Film Festival. The Sunday Herald is the festival's media partner. For programme and ticket information visit https://glasgowfilm.org/glasgow-film-festival

The film will be released in the UK in April