The nerd’s a superhero!

Parsons has something you only see in comedy greats such as Arthur Lowe, Alan Rickman, Hancock: he makes you care about the most disgraceful bastard.

As Big Bang’s alpha nerd, Sheldon Cooper, he is all Star Trek T-shirts and crashing rudeness. (Sheldon has a fictional restraining order from the late Leonard Nimoy.) In real life, perched on a chair in Claridge’s, the slim, tall Texan is a creature from the past, a wildly charming Noël Coward hero in elegant grey shirt and tie. Even his speech patterns are retro: Twitter he “doesn’t care for”.

He’s admiring his plush hotel room, not trashing it, because not all his British trips were like this. Only a decade ago, he was in a hotel in Milton Keynes, down to a handful of coins, reliant on a phonecard “to call my friends, as I was lonely”. He had managed to get a role in a fast-food ad, and not just any vaguely embarrassing job. In the ad, a man on a park bench asks him why he isn’t eating the special kind of toasted sandwich he himself is chomping on. “What were you, raised by wolves?” Parsons’s character thinks back. Cut to him writhing in the dirt, being breast-fed by a “wolf”.

“The wolf was played by a husky, and they made a fake nipple for me to put my mouth on,” he says. He can’t forget the smell. “Oh my God, the indignity.” Did he actually... “God, no!” He didn’t need to. “How low can you get?”

Lower. It came time to check out of his Milton Keynes hotel. “I didn’t understand how phonecards work, and when I checked out, they showed me the bill. It was something like £70. I didn’t have £5. I literally burst into tears. They just let me go. That was sincere bottom of the barrel. I never went without food, but it was rope to rope.”

Then he got his hands on a few pages of a new script about two geeks and a girl. He read Sheldon’s lines and instantly sensed what made him funny: confusion. “What I loved were these comedy rhythms in the speeches, little puzzles to figure out. I worked hard memorising it, because I knew that it wouldn’t be funny at all if it were laboured.”

For three days, he worked as his mother had taught him, drilling the pages of science jargon. “I’d put a pencil between my teeth because it was really hard to articulate. Then I went in the room... Chuck was there.” And Lorre laughed. From that moment, Parsons’s place in TV history was assured. Today he earns a million dollars an episode.

He writes himself off as a “decently good” schoolboy whose piano and saxophone lessons got him out of sports. However, growing up on the American versions of British sitcoms, something clicked. Alf Garnett’s popular US incarnation, Archie Bunker, and the American George and Mildred Roper spoke “a language I felt like I learnt to speak from watching it. I enjoyed music, and there’s undeniably a rhythm to the multicamera, in-front-of-the-audience sitcoms.”

Sitcom sort of entered his DNA. “Without knowing it, I understood a cold open [pre-credit gag scene] and scenes through Act I and into Act II and the tag [post-final-credits gag scene]... I’ll say a line in the rhythm I know it needs to be before I understand the meaning. And having to spit it out at that rate makes that emotion. I love it.”

While he was studying drama at the University of San Diego, acting in comedy fell into place for him — interplay, not stand-ups doing stand-up at each other — but then came frustration. He was gutted to lose the role of lemonade-sweet Kenneth the page on the TV spoof 30 Rock to Jack McBrayer, until he saw McBrayer’s skill. “If you’ve got that, you don’t need my services!” he laughs. (And, of course, 30 Rock went the way of most arch single-camera comedies shot drama-style without an audience: not enough people liked it. The Big Bang Theory is now in its eighth season.)

Then came the part of a lifetime. “There is no more joy than playing someone who is honestly obtuse,” he says. Sheldon Cooper doesn’t get it. Many comic characters don’t get it. But only the great ones think everyone else is the problem. From Melchett in Blackadder Goes Forth to Niles Crane in Frasier and Captain Mainwaring in Dad’s Army, it’s the secret of a great clown. This is what Lorre calls “the egotistical glory” of Sheldon: he’s an ass.

In a notorious episode, Sheldon is baffled to be summoned by HR for telling a co-worker: “A woman is like an egg salad sandwich on a warm Texas day. Full of eggs and only appealing for a short time.” He walks through the door to find the actress Regina King, queen of the deadpan — not just the HR boss, but black. “Oh! I see the confusion here. I meant that all women are slaves to their biological urges, you know? Even you. You’re a slave.” “I’m a what?” Regina says. The audience gasp as King stares for one of the best beats in comedy, until Sheldon explains female reproduction with the patience of a saint.

Finally noticing he’s in trouble of some kind, Sheldon tells on all his colleagues like a five-year-old. “And I heard Rajesh Koothrappali refer to you several times as Brown Sugar!” Adding, helpfully: “That wasn’t racist, he’s also brown.”

“It had nothing to do with her!” Parsons says, suddenly in character, furious. Then he cracks up. “And I have to tell you, had it not been Regina King, one of the nicest people we’ve ever had on the show, I don’t know that I could have done it.”

Good clowns have an instinct for balance. Matt LeBlanc spotted that the original Joey Tribbiani was the wrong kind of ass, a playboy ass who could break our favourite characters’ hearts. Fearing he wouldn’t last the series, LeBlanc asked nicely if it would be OK if his character didn’t bed Monica and Rachel, just most other women — and Joey lasted the full 10 seasons of Friends.

Only Sheldon could boast of his (imaginary) sex life with his long-suffering girlfriend, Amy (the hysterical Mayim Bialik), “What can I say? She enjoys my genitals.” To be this rude, you have to be smaller in other ways. Parsons plays Sheldon as a terrible toddler you want to hug. The only person who gives him respect? “Amy,” he spits childishly, Sheldon again. “But she tests her boundaries with her wanton sexuality.” Amy, for the record, is written with the wanton sexuality of Angela Merkel: perfect balance again.

Parsons shines in front of live audiences, too. He starred in a reprise of James Stewart’s role in Harvey, played an Aids activist in the Broadway (later HBO) hit The Normal Heart, and will riff as the Almighty in the forthcoming comedy An Act of God. He also manages without even a cast to play against in the animation Home, based on the children’s book The True Meaning of Smekday by Adam Rex. A girl (Rihanna) meets an alien species called Boovs that prides itself on its cowardice. Parsons plays the odd Boov out, and together they go on a quest. It has Steve Martin as the baddie, jolly Rihanna tunes and jaw-droppingly stunning animation. Parsons is, predictably, terrific in it.

He’s still star-struck by the (disappointingly) undivalike Rihanna, “funny, warm and quick”, and the (disappointingly) uncrazy Steve Martin, “The complete opposite — professorial, he has his glasses and his paper.” He came to work one day to find “secret service men crawling around the whole place” before being presented to Obama. As knackered as he looks? “No! Completely comfortable... a very reassuring thing to see in a president,” he deadpans.

At the top, comic genius plus vast sums of money is a dangerous load. The last star to be as big as Parsons, Charlie Sheen, the original star of another Lorre hit, Two and a Half Men, had a conversation with a few kilos of crack cocaine, which told him that while he was a comic genius, Lorre was the devil. (In the early days of television, this conversation would have been held in a poolside cabana at the Chateau Marmont, the only witness a housekeeper called Conchita trying to vacuum under the prostitute.)

Parsons is a very different animal, analytical enough to see through the unknowns of comedy, domestic enough to protect himself from the emotional. He doesn’t drink and is in a 13-year relationship with someone his own age, not his shoe size; his other half is called Todd, not Ginger or Capri. The gossip ends there.

“We were set up on a date... I thought he was too good-looking for me, and I’d have to be charming in some way, because no one was going to look over at this,” he says, indicating his face. Their friends nailed it: “TV’s cutest couple” even co-ordinate natty suits.

Gay liberation has bowled Parsons a googly. He now faces the same grilling straight actors have had for decades. Will he marry because the gay chat-show host Ellen DeGeneres told him to?

“No! I don’t know... It’s such a weird situation. Something that was not even a possibility has become one. But we have a wonderful time,” he says of their perfectly uneventful home life. “I get huge joy crawling into bed at eight so I can read.” He has discovered the alternative to Sheen’s talking crack: a good book, “disgusting-sounding cosy mysteries”, PD James and Agatha Christie offering “small-town insular mysteries I find such comfort in, so separate from everything going on in life”. Then he gets up at 5am and does it all again.

Home opens on March 20