The couple have daughters Clara, 16, Esther, 9, and adopted Jamiyan, together but the 40-year-old Scottish actor said that despite having spent more than a decade together, they still don't know everything that there is to know about each other.
He told the Daily Record, 'I still feel we are beginners.
'I've been in my relationship for a very long time. There are always new things that happen - things that surprise you or you discover about yourself and your partner all the time.
'However the feeling, the lovely feeling of being with someone for a really long time, it's not about being a beginner - it's the fact you know each other so well and you're so comfortable in each other's company.'