Sam Rockwell is still recovering from his temporary insanity, brought on by the grave responsibility of starring in movie beast The Hitchhiker’s Guide to the Galaxy.

He plays Zaphod Beeblebrox in the film, and was determined to turn the beloved character into much more than the President of the Imperial Galactic Government and owner of three arms, two heads and one planet-sized ego. So he lay on our shrink’s couch to explain why the film gave him an unhealthy obsession with rockers and presidents, why his ex-girlfriend quickly shared his compulsion and why he struggled fighting his stormy temper to play the eternally cheerful alien.

Q: Why did you turn down the legendary part of hitchhiking alien, Ford Prefect in the film, and ask to play Zaphod Beeblebrox instead?

I read for the part of Ford Prefect several times and then after reading the script, I said, "Hey, what about this Zaphod guy?" I wanted to play him more because he’s more flamboyant and fun. But I was worried because the producers were thinking about getting a comedian to play him. But I read (director Garth Jennings) a line of Zaphod, really badly, like Garry Busey. I read everything like Garry Busey, even the Scottish play. But for some reason they gave me the part.

Q: Garth Jennings said he was amazed by how obsessed you became with the role. What did you do to shock him so much?

Well I became obsessed with the book and getting it just right. And I even met up with Mark Wing Davey, who played Zaphod in the TV series to see what he had to say. But what Garth’s probably talking about is the fact I watched footage of presidents and rockers all the time. Zaphod’s the most popular, arrogant, flamboyant guy in the universe. So Garth would find me in the studio really early, in my wig and fake tan, studying footage of Bill Clinton, George W Bush, Freddie Mercury and Elvis Presley.

Q: Is it difficult watching the movie now, and remembering how much your ex-girlfriend, actress Gina Bellman, helped you develop the character?

I was in a very serious relationship with Gina Bellman. And she would go through the script with me all the time, and come up with cool ideas. It was Gina who suggested I look at Freddie Mercury footage for inspiration, so I’m very grateful. She’s great. She also gave me the idea of basing some of Zaphod on my friend Vince Vaughn.

Q: What did Vince Vaughn think about you basing an arrogant, dim-witted character on him?

Well, Vince and I would meet up and imitate Elvis Presley, and it’s sort of a mixture of Vince and Elvis which I showed to Garth Jennings in my first reading, and it got me the gig. Vince thinks it’s great.

Q: It sounds like you felt the great responsibility of doing justice to Douglas Adams’ legacy. Was it a tense time for you?

Well, there was a moment in rehearsals where there was a bit of panic because I couldn’t decide how to play the second head. I considered giving it a New York accent, but that wasn’t working. I realised I’d gone very Bill Clinton with the first head which had made him too slow, so I had to speed it up and make the second head incredibly hyper.

Q: Your co-star, Martin Freeman said he pitied you, because unlike him, you had to be in a permanent good mood to play an eternally happy alien. Was that difficult?