In any kind of comic scene you're going to perhaps push the boundaries of plausibility but as long as there is some semblance of logic I think as an audience you'll buy it and as an actor, when it comes to playing things like that, it gives you something to delve into. When I don't buy into a comic scene is the type of scenario where you'd just go: "Well, that would never happen."

I was nervous about playing a lead part in a Working Title romantic comedy and I was also nervous about the fact that I not only had to take my clothes off, but get my willy out. There's certain things you can do to make yourself look better, but there's nothing you can do about your willy. Your willy is your willy and no amount of working out is going to make your willy look any different. You get what you're given. But I wanted to look my best and to whip myself into any semblance of handsomeness. And that was hard going.

I'll probably always be 'Timothy Spall's son' and it's something I'm proud of. Maybe one day as well as that, they'll say of Timothy Spall that 'He's Rafe Spall's dad'.

Because I was always a fat child, I got fatter and fatter, and I ended up 18 stone and with a 40-inch waist.

I watched what I ate for a bit and did a bit of exercise. I wanted to look alright because I knew I had to take my clothes off and I knew I was starring alongside some extremely attractive women! I think it was Humphrey Bogart who said that the only reason people found him attractive was because of the attractive women he played opposite. So, as audience member you go: "Well, if she would find him attractive, then surely I must too!" Playing opposite beautiful actress of the calibre of Rose Byrne and Anna Faris is amazing - it does a lot of the work for you.

I love doing comedy, I really do. It was perhaps my first love. And I think, as an actor, you're young and you do school plays and the reason you go 'I might do more of this' is because you make people laugh in a school play. You don't go and do Hamlet when you're nine and go: "I feel people were really moved out there!" You do a silly voice and everyone laughs and you go: "Ooh, that feels quite nice. I might make a life out of this!"

Doing Prometheus was what you imagine being an actor is like when you're five. In a spacesuit, on another planet, getting killed by an alien. It was a real treat, it felt like being a part of movie history.