Every actor needs a defining role early in their career. For Josh O'Connor, it was playing the taciturn farm hand Johnny Saxby in Francis Lee's 2017 award-winner God's Own Country. Since then, his career has exploded, from being nominated for a BAFTA Rising Star award to winning roles in everything from the recent TV version of Les Misérables to the forthcoming season of The Crown. So how does it feel to be in such demand? 'I pinch myself every now and then,' he admits, blushing.

Fortunately, this down-to-earth Cheltenham lad hasn't let it affect his work. His latest role, in Harry Wootliff's intimate and truthful romantic drama Only You, is every bit as splendid as his performance in God's Own Country. He plays Jake, a 26-year-old PhD student living in Glasgow who meets office worker Elena (Victoria star Laia Costa) quite by chance on New Year's Eve. A relationship swiftly sparks, as they fall in love, but with Elena almost ten years older, her biological clock is ticking.

Unlike the typical Hollywood fantasy that sees couples get pregnant instantaneously, they struggle to conceive. 'This was a couple who fall madly in love very quickly, and then it's a case of this other thing, wanting a child, that stunts it and interrupts it,' says O'Connor. 'I found it really strange because there were times when I felt there was this third character, the child that we wanted.' Gruelling rounds of IVF follow as the stress plays havoc with their relationship.

Noting that real nurses were used in the IVF scenes, O'Connor deliberately steered clear of learning too much in advance about the subject. With the film shot chronologically, 'it was quite nice to feel and experience that as we went along,' he says. If that contributed to Only You's spontaneous air, O'Connor and his co-star assuredly nail the pain and anxiety of trying to get pregnant. As he puts it, '[You see] the stepping stones of hope that come along the way but then can crash … and then do you keep going?'

O'Connor, 28, was particularly taken with filming in Glasgow's hip Finnieston area. 'I loved my time there,' he says, 'and it was perfect for the film. The thing you notice … it really is such a cosmopolitan city. Someone like [the Spanish] Elena would just exist in that world. There's a lot of students in that town, but also a lot of people from all over the place. I really liked that Harry wasn't really commenting on the fact that Elena was from where she was.'

Since Only You, O'Connor has worked non-stop, from shooting the fourth and final season of ITV's The Durrells to co-starring with Annette Bening and Bill Nighy in family drama Hope Gap. Most significantly, he's now filming the third season of Netflix phenomenon The Crown, playing the young Prince Charles opposite an 'incredible' Olivia Colman's Queen Elizabeth II. With 'proper pros' like Helena Bonham Carter and Tobias Menzies also cast, O'Connor can't wait for it to be unveiled. 'I'm pretty excited about what we've done together.'

Only You screens at Glasgow Film Festival on Fri 22 Feb.