Robert Pattinson is tired and worried. Tired because he's been up all night shooting some of the final scenes for the last part of the Twilight saga, Breaking Dawn - he worked from sunset to sunrise, as seems appropriate for a man inhabiting the character of a vampire who's over a hundred years old – and worried because of the hysterical coverage he's been watching on US TV of the royal wedding.
Given his own experience of being catapulted from a life of quiet, middle-class obscurity to global fame in one box-office weekend, he can't help but wince at the similar fate befalling Prince William's wife, Kate Middleton.
"It's going to be hard, because people will lay responsibilities on her that will seem totally irrelevant," sighs the 24-year-old ex-Harrodian, who has turned up to his Radio Times interview at a Santa Monica hotel wearing the kind of gawky outfit - easy-fit chinos, blazer, trainers - that no self-respecting vampire would be seen undead in.
"You can't mess up, either," he adds, darkly. "As an actor, you can kind of mess up - but not if you're a royal. I've always liked the members of the royal family who couldn't care less what anyone thinks: the ones who go, 'I'm royalty - so shut up!' That's one of the coolest things about England, I think, that we still have this crazy old system in place."
It probably shouldn't come as a surprise that marriage is on Pattinson's mind: the recently filmed wedding and honeymoon scenes between Edward Cullen (his Twilight character) and Bella Swan (played by Kristen Stewart, who's rumoured to be his girlfriend) have just been leaked on the internet - months before their official release - prompting an uncharacteristically vicious response from the usually low-key English star.
"You're not a fan, you're just a d**k!" he said of the unknown perpetrators, before calling on his millions of "Twihard" groupies to "Find out who these little s***s are and hack into their computers."
The irritation has no doubt been heightened by continuing speculation over the nature of his relationship with Stewart, who was raised in Los Angeles and is four years his junior. Pattinson tends to just mumble and shrug when asked directly about this, and his studio handlers make a point of warning journalists not to even think about bringing up the subject in interviews.
Alas, this strategy has served only to encourage anonymously sourced rumours that the pair are "on a break" and "just mates" after "drifting apart", especially following Pattinson's long shifts working on Water for Elephants. Although pictures of the two cosying up to each other at the New York premiere will no doubt fuel further speculation.
Not that Pattinson has any complaints about his schedule. "Twilight was very much a fluke for me," he says, modestly, "so getting a job like Water for Elephants, with people like Reese [Witherspoon] and Christoph [Waltz] - it's just a massive payoff."
The role came up when he was introduced to the director Francis Lawrence (I Am Legend), who was already planning to adapt the bestselling novel by Sara Gruen about an orphaned Depression-era veterinary student who escapes his debts by joining the Benzini Brothers circus.
"I didn't audition or anything," says Pattinson. "I just talked to Francis, and it felt like a good decision, even though I wasn't even vaguely intending to do another romantic movie or a period drama."
One other good reason for accepting the part was that, with his co-star Tai, there wasn't going to be any speculation about an off-camera romance. After all, she's 42 years old... and 650 stone. Which is quite slim for an elephant.
Pattinson isn't entirely without a love interest in the film, however: he has several intimate scenes with Witherspoon - even though as a 17-year-old he was cast as her son in Vanity Fair. "It was one of my first castings, and I was looking at Reese in front of me, thinking, 'You look 25 years old, and I look about 21', " he recalls, laughing.
"It was crazy." Just to make things worse, he forgot his lines, the scene was cut and Witherspoon now claims that when Pattinson signed on to Water for Elephants, she had no idea she'd worked with him before. Meanwhile, as shooting began in a custom-built circus tent near Piru, California, yet another awkward situation arose between the Southern Baptist girl and the public schoolboy from Barnes: Pattinson caught a filthy cold.
"It was terrible," he groans, head in hands. "I had the most disgusting infection, ever - and I love how Reese brings that up whenever people ask her if the kissing was sexy or not. She's like, 'Oh, it wasn't just unattractive, it was absolutely revolting, the worst experience of my life!'"
Of course, this might also have something to do with Witherspoon not wanting to upset the powerful Hollywood agent Jim Toth... whom she married a few weeks ago.
In spite of the Twilight movies having earned close to $2 billion (£1.2 billion) at the box office, Pattinson still likes to give the impression he doesn't quite belong in Hollywood. "I wasn't into drama at all when I was growing up," he insists.
"I just went to this theatre group around the corner from my house and worked backstage for a couple of years. [His mother had a job at a modelling agency; his father imported vintage cars from America.] I really liked the people who went there - that, and the fact it was a way to get into the pub despite being underage."
All of which led to his botched cameo in Vanity Fair, followed by a part in the awful Curse of the Ring (he was overdubbed) and then, finally, his break as the doomed Cedric Diggory in Harry Potter and the Goblet of Fire. In spite of the latter role, it didn't take long for Pattinson to become disillusioned with Hollywood.
"When you first move to LA, you're like, 'Wow, everyone's so smart! They all know me really well!' Then you realise they're just fantastic bulls**t artists."
It wasn't until almost three years later that he was cast as the lead in Twilight. If there's any downside to Pattinson's early success - aside from his inability to do anything in public without being mauled by teenage girls - it's that expectations for his career are now almost impossibly high.
And yet he has to move on, not least because filming of the saga has now pretty much wrapped (Breaking Dawn will be released in two instalments, this November and next).
"It makes you think differently," he says of being part of such an incomprehensibly lucrative venture. "You see a lot of people who do a big Hollywood franchise, then afterwards they're asking for $10 million pay cheques and thinking that's their value. That's dangerous, especially when you're young.
“The other thing is, as soon as you do a movie that makes loads of money, everyone judges your movies by how much money they make. Whereas if your movies never make money, they just judge the movies. It's an annoying little conundrum."
When asked if he views this as a curse, he deadpans, "Not really - I'll just keep making a ton of movies that don't make any money. They'll ask me, 'So why did you choose that role?' and I'll go, 'Well... because I thought it would be a total commercial disaster.'" The thought makes him laugh.
In the meantime, Pattinson has been writing his own screenplay - an adaptation of the politically incorrect Martin Amis novel Money about a chronically overweight, lecherous, hamburger-addicted director of 1980s fast-food advertisements who tries to break into Hollywood. The only problem: BBC Two debuted its own poorly received version of the cult book last year.
"I've wanted to make it into a movie for so long," sighs the actor. "I finally got the perfect little structure for it, then it suddenly came out, and I was like, 'Why?! Why now?!' I've been shopping it around, not that anything's come of it yet.
“The thing is, you can't do it as just the story, that's what [the BBC version] seemed like. The whole point is the writing, and you've got to find a way to translate that into film. It would be such an amazing movie, though. The dialogue is all there."
As for Amis's scathing portrayal of Hollywood excess - the British novelist worked briefly as a screenwriter in 1980, providing the script to the awful sci-fi drama Saturn 3 starring Kirk Douglas and the late Farrah Fawcett - Pattinson says, "That was definitely a thing in the 1980s, you can completely believe it. In fact, when you meet producers who were big in that decade, they still behave like that. But nowadays they've become quite sophisticated in their bulls**t."
Pattinson is under no illusions about the brutal economics (not to mention politics) of Hollywood, in particular the fact he must do big studio films to win the freedom for smaller, artier roles. "As with theatre in London, there's an accepted protocol, and if you start screwing with it - if you don't kiss certain arses and stuff - it comes back to bite you, and it's quite difficult to keep working," he muses.
Which means he won't be taking a break in England any time soon, even though he hasn't been home for more than a year. "I miss it a lot sometimes," he says. "Mainly it's that English celebration-of-the-underdog thing - like, 'Yeah, we can't do anything, but we're still better than everyone else.' I love that attitude, you never see it in any other country."
"Nevertheless," he adds, "whenever I go back, everything's totally different, anyway. Most of my friends have moved somewhere else, or they've established totally different lives for themselves – so in a way, it's really kind of horrible."
Of course, there is one sure-fire way to relieve his homesickness while on the wrong side of the Atlantic: "Yes, I'll definitely be watching the royal wedding," he confirms, a little sheepishly, a couple of weeks before the event. "I mean, they're absolutely obsessed with it over here, it's insane... but I guess it is kind of a big deal. Actually, I think it's really sort of fascinating."
A bit like a certain other relationship involving a young Briton from a modest family who suddenly becomes a global news phenomenon, perhaps? Pattinson doesn't even have to say the words. No comment.