One of the most versatile actors of his generation, Rafe Spall discusses the pros and cons of his father Timothy being a national treasure, and claims that failing his drama GCSE was just a bump in the road

I grew up in a big house in Honor Oak Park in south-east London that was often full of actors. Every Sunday, my parents would invite the likes of John Sessions and Frances Barber around for lunch, followed by a big dance-up. My two sisters and I loved it, though most of the time we just watched everyone get tipsy and have fun. I wasn’t a gregarious kid, but I had a golden childhood. My parents always encouraged me to be whatever I wanted to be.

Confidence is one of the greatest gifts you can give to anyone. I hope to pass it on to my two young children. As is self-belief. The strange confidence instilled in me by my family – strange because it really was unfounded when I was younger – has meant I’ve always felt I had a shot at making it as a professional actor. Not because I was extraordinarily talented or brilliant, but simply because I was willing to give it a go.

Most actors who fail their GCSE drama and are refused entry to RADA – where my dad, Timothy, had been awarded the Bancroft Gold Medal as most promising actor in his year in the Seventies – would have called it a day. But for me they were just bumps in the road. I did, however, manage to get into the National Youth Theatre. I rehearsed a speech from Julius Caesar as my dad sat on the sofa, drinking white wine. I felt sick and I think he did too. Me, because I was reciting Shakespeare in front of one of the best actors of his generation who happens to be my dad, and him because his little boy was doing a speech in front of him. It was distinctly embarrassing.

I did a play in the West End last autumn called Constellations with the brilliantly funny Sally Hawkins. When my dad came to see it for the third time, he mentioned that Julius Caesar audition; he said he knew I ‘had’ something back then because of some silly voice I’d done.

We don’t share ideas about the nuts and bolts of acting, but we do give each other advice about roles. I have learnt from watching him: I do a massive amount of work ahead of a job. A huge amount of reading and any physical preparation that might be needed.

I do this not because I’m a swot, but out of fear. I don’t want to embarrass myself on set. Fear of failure pushes me forward. I feel constantly guilty preparing as I think you can never do enough. But, once I’m on set I can throw all that baggage out of the window and be inventive. I like to keep it fun and light on set right up until the director shouts, ‘Action!’ because I think it’s important to be relaxed. Relaxation is very important to the way I approach acting, so I’ll sing songs, be silly.

Which is not to say I don’t take the job seriously. I truly love acting. I love watching it close up. I can be so moved by a performance. It’s easy to sound cheesy when you are talking about acting, but when it’s done properly it’s pure. It can be a true mirror of how we feel. Some actors gravitate towards playing a version of themselves on screen while others change dramatically from one role to the next. Both are equally difficult to do. Look at Hugh Grant: people say he’s just playing himself, but it’s not as easy as it sounds. To bring whatever you think makes you interesting, clever, charming and original in everyday life and translate it on screen is a huge challenge. Few people do it well. Hugh Grant is one of them.

I suppose I play more of a version of myself in I Give it a Year than I did in Life of Pi or Prometheus, although my character is sillier and a bit of an idiot. I had to work to get into the sort of physical shape where it was feasible that an actress as hot as Rose Byrne, my on-screen wife, would fancy me.

Dan Mazer, the director of I Give it a Year, wanted me to play the most handsome version possible of myself. It took considerable effort on my part. I worked out at the gym regularly and a woman would come over to my house to give me a fake tan and a facial once a week. I think I looked OK in the end.

We did a huge amount of improvisation. Part of the material was found on the day. Improvising comedy is scary: the camera was always rolling and Dan never called ‘cut’. Comedy is black and white; it’s either funny or it ain’t. To arrive at something funny you have to throw all this material out there and be prepared to make a complete fool of yourself. And you’re thinking all the while that this is a Working Title romantic comedy, and that’s a big deal. A really big deal.

I’ve had some amazing experiences on set in the past year. At the risk of sounding earnest, I felt like I was breathing rarefied air when I worked with Ang Lee on Life of Pi. They had already shot the film with another actor playing the part of the writer when Ang Lee decided he wanted someone who was less recognisable. So they went with me.

Ang Lee is an extremely specific director who knows exactly what he wants. Towards the end of the shoot he asked me to say one line 88 times in a row. He kept giving me notes: do it like Ernest Hemingway. Do it as though you’re telling God that you believe in him.

It took an extraordinary level of concentration on my part. My torso was drenched in sweat by the end of the shoot and I had to change my T-shirt. But it was worth it.

I was delighted my dad liked Life of Pi. Any young actor being told they’re not at all bad by their dad is going to experience a boost. Any young actor being told they’re not at all bad by Timothy Spall is definitely going to feel proud. Imagine they’re one and the same: you’ve got this incredible double whammy of encouragement.