My earliest memories are colours. I remember the yellow of my cot and a green glass jug that we had on the kitchen table. Colours, music, smells –they’re all time machines. I turn 50 this year, which has always been my scary age. But I’ll have a word with myself because everything is relative. You can be a miserable 23-year-old, then be pants-in-the-air happy at 50. I suffered from impostor syndrome when I was starting out. I come from a working-class background and often felt like I didn’t belong. That never leaves you completely. Even the most successful actors have this constant worry that a timer is about to go off and the jig will be up. The most annoying people have the loudest voices. Unfortunate but true. It’s become even more apparent in recent months. Having a passion feels like a superpower. I was teased a wee bit in my teens for being creative and a bit different but I knew what I wanted to do. I had this precious thing. That gave me a defence mechanism and sense of identity. I only get starstruck around musicians. If I’m in the same room as Elvis Costello or Rufus Wainwright, I want to fall off my chair. I never approach them – I don’t want to burst that bubble. I love how fandom makes you feel 15 again. If someone feels “other”, I don’t want to pedestrianise them. The last time I cried was yesterday, when my brother phoned. He has Alzheimer’s and is in supported care but was feeling a bit lost and confused. It’s tough right now for people who are vulnerable. Just turn up. That’s what my mum used to say. Don’t cross the road to avoid someone in trouble, upset, separated or bereaved. Just be there, even if you don’t know what to say. I call myself “Duff” when I’m cross. I have no idea I’m doing it, but if something goes wrong in rehearsal, I’ll say, “Come on, Duff!” like I’m a racehorse. My most embarrassing moment involved Prince Charles. I was at a premiere with my ex-husband [actor James McAvoy]. Prince Charles said, “I suppose you two never see each other.” I said, “We see each other in bed.” Love is the key to happiness. Coming from a very poor background, I know that money makes life a lot easier. But love is more important. To know that you’re loved, to know you’re worthy of love, to have love around you, to give love – that’s all we can hope for. Happiness is fleeting. There are hills and valleys. But if you can lean on love, then you’re safe. The Salisbury Poisonings starts on Sunday 14 June at 9pm on BBC One and airs across three consecutive evenings