Colin Firth, 33, is the eldest of three children, all of them actors. After graduating from the London Drama Centre he was offered the lead in both the stage and film version of Another Country (1983). He played John McCarthy in Hostages and from September 2 appears in Master of the Moor, a Ruth Rendell mystery on ITV. He is based in London, but spends time in Vancouver, where his son, Will, 3, lives with his mother. Kate Firth, 31, was a birth control counselor in California until two years ago, when she left America and joined the Bridge Theatre Company in Sherringham, Norfolk, subsequently receiving rave reviews as Hedda Gabler. She has also studied holistic massage and is now a licensed practitioner specialising in creative performance.

KATE: Colin and I used to dress up, I was the princess in jumble-sale ball gowns, he was the prince in cloak and breeches. We were competitive but also protective of one another. At infant school he gave me instructions not to leave until he collected me from the classroom. At break he always made sure I had enough money to buy a Thunderbirds chocolate bar.

As teenagers, I was passive and Colin was noisy and extrovert. I felt I had to be safe and conventional and was jealous of his arty, bohemian friends. I looked up to Colin, who was much more competent and confident than me. With academic parents, it was expected that we would go to university. Colin could have gone if he'd wanted to, but his heart was set on drama school. I desperately wanted to act but never had his courage. For me, drama ended up as part of a university degree course. It was a world that no one in the family knew anything about. Our parents didn't think that success as an actor was a real possibility, but Colin and I shared a strong conviction that one day we would make it.

I knew Colin was talented because I'd seen him in school plays and in productions at the Drama Centre. In my first year at London University he did Hamlet. I sat there mesmerised, feeling terribly proud and jealous at the same time. It wasn't horrible, destructive jealousy, it was just that he was doing exactly what I wanted to do. After that his career took off. It's an odd experience watching Colin act. He really becomes the character. Most of the time I forget it's him, then suddenly I see a smile or hear a tone of voice and think: "I know that person".

We only came to know one another properly as adults. After university I married and moved to California. Colin was in Vancouver but visited LA for work and stayed with me. Away from our roots, we could be rude about America and England without hurting anyone. Before I decided to give up my life in California I went through a bad time and Colin was very supportive - a good shoulder to cry on. Emotionally he was going through his own crisis, but like most men, he doesn't open up much.

Since he became a father I've noticed a much gentler, softer side to Colin. He absolutely adores Will but had to choose between family life in an isolated log cabin and working. Colin's a city person who needs his friends - people he's been close to since he was at school.

He helped me break down the script of Hedda Gabler into manageable goals. He was a brilliant teacher - encouraging and constructively critical. His way of saying I was overacting was to tell me "Well, you could do less." I've always admired Colin's integrity and sincerity, and wouldn't ever want to trade on his name.

In spite of all he's achieved, I sense Colin still feels he lost out by not going to university. I don't understand his insecurity because in a group of half a dozen people he'd be the one we were listening to. I've never won an argument with him in my life.

COLIN: Kate was born in Nigeria, where we lived for three years. I remember sitting on an African rug, waiting for my mother to position me and my baby sister for the photographer. Back in England I expect I was pretty nasty to Kate. I was naughty and she was sweet-natured.

Academically, Kate was much brighter than me. Arrogance got me through school. Compared with Kate I felt like the black sheep. I was always conscious of her excellent school results and spectacular reports. I wondered how she did it. I had the intelligence but never worked out how to do exams.

It mattered what the family thought when they saw me at the Drama Centre, and I particularly cared about Kate's opinion. Two years ago, when I read reviews saying she shone as Hedda Gabler, I was over the moon for her. Even though I knew she would be good, there was still an element of surprise when I saw her on stage. I hadn't expected that authority and conviction. I know Kate pretty well but I certainly didn't know this Kate. Last year I saw her play Narissa in The Merchant of Venice and her performance really knocked me out. In acting there's no middle ground. The stakes are very high and I was fearful for her. I would be fearful for anyone I was close to who took the leap Kate did. I've been there.

Even though she's been back in England for two years, we haven't seen much more of each other than when she was in California. In some ways it was easier to make time for each other there. We were so far away from the old territory and familiar characters that we had real freedom to develop our relationship as adults, not simply as brother and sister. Neither of us would ever want just to let things be on a superficial level. But when we were both going through difficult times, I found it almost impossible to cry on her shoulder. She's almost 32, but she's still my little sister. I can't cry in front of anyone. I keep the tears for acting.

Kate is very open about what concerns her. It was horrendous for her to make the decision to come back to England. I wouldn't give her any tips about the business unless she asked, but it was interesting going through Hedda Gabler with her. Rather than give advice, I'm more inclined to issue dire warnings. It would be unfair of me to encourage her unconditionally. Acting plays havoc with your personal life.

Kate is more honest, less selfish and braver than me, which isn't to say she's an angel. She can be aggressive, determined and tough. She's very territorial and needs space, and she needs a degree of distance from me. I hope she has peace of mind. Not many people in the business are lucky enough to find the balance between a productive personal life and a productive professional life, but it's the least I would wish for her.