THE passage of time is perhaps not so acute to the centuries-old alien at the heart of the BBC’s “Doctor Who,” the shape-shifting hero known simply as the Doctor, who has had more than 50 years of adventures across dimensions known and otherwise.
But time and its measurement have become especially crucial to Peter Capaldi, who will make his proper, full-length debut as the latest actor to play the Doctor when the new season of “Doctor Who” has its premiere Saturday on BBC America.
It has been just over a year since the BBC announced that Mr. Capaldi would succeed Matt Smith, who was a 26-year-old relative novice when he was chosen to play the Doctor, and, after three seasons of putting his frantic, whirling-dervish stamp on the character, disclosed his departure in June 2013.
Since then, Mr. Capaldi, 56, has spent several months filming “Doctor Who” in Cardiff, Wales, trying to bring to the role his own personal take, which he says is more sardonic and elusive.
Still, as a lifelong “Doctor Who” fan, he could not quite contain his giddiness, all this time later, that he had actually landed the part.
“I just didn’t think that they would be going in this direction,” Mr. Capaldi said in a gentle, stately voice with only traces of a Scottish accent, on a visit to New York last week. Asked what he meant, he answered with a laugh: “Well, I guess, older. And more like me.”
Mr. Capaldi’s penetrating eyes and expressively lined face will be familiar to viewers of Armando Iannucci’s 2009 film satire “In the Loop,” and the BBC comedy that spawned it, “The Thick of It,” in which he played Malcolm Tucker, a short-tempered political aide who fired off obscene insults as fluidly and creatively as Shakespeare composed sonnets.
But he is still untested as the Doctor, particularly with American audiences who responded strongly to Mr. Smith: BBC America said the series grew from an average total viewership of 910,000 in his first season to nearly 1.9 million in his last, a trend the channel would surely like to see continue under his successor.
Though Mr. Capaldi is among the more accomplished actors to take on this storied science-fiction franchise, he is hardly a relic. But his age nonetheless represents a departure from the recent history of the series.
It is one more unknown factor for producers and audiences alike to consider as “Doctor Who” begins a crucial transition that elicited passionate criticisms and defenses before Mr. Capaldi set foot in front of the cameras.
“When launching a new Doctor, I don’t want to make it sound as though he’s just one of a set of options,” said Steven Moffat, the executive producer and lead writer of “Doctor Who.” “He’s the one and only right now.”
Mr. Capaldi is playing the 12th canonical version of the Doctor, though the show cheekily acknowledges its counting system has run off track a bit.
Born and raised in Glasgow, Mr. Capaldi grew up admiring character actors like Peter Cushing and John Hurt, and was a follower of “Doctor Who” more or less from the start.
Between the 1960s and 1980s, he watched the series transfer its lead role from elder statesmen like William Hartnell to expressive wits like Tom Baker, and, in its 21st-century revival, heartthrobs like Mr. Smith and David Tennant. (Mr. Capaldi also appeared in a 2008 “Doctor Who” episode playing a Roman in ancient Pompeii.)
As a steadily employed actor, Mr. Capaldi said, he’d fallen into a routine of “increasingly bland parts, turning up in episodic television as the slightly untrustworthy doctor or shrink, or the M.P. with a gay secret.”
“That was fine,” he said, “but quite dull.”
That changed in 2005 when Mr. Capaldi met Mr. Iannucci, creator of “The Thick of It,” on a day when Mr. Capaldi had come from another demeaning BBC audition and was not in a particularly good mood. “I was like, ‘O.K., show me what you’ve got,’ ” Mr. Capaldi recalled. “It was lucky I had just the right attitude at that moment.”
Mr. Iannucci, the creator and show runner of HBO’s “Veep,” said he recalled Mr. Capaldi as initially “very amiable and softly spoken.”
“When the switch came,” Mr. Iannucci said, “from this personable charmer to this rather ruthless and cold, frighteningly still person, I thought, ‘My God, that’s quite a trick you can pull off there.’ ”
Mr. Moffat said that the casting of Mr. Smith and Mr. Tennant on “Doctor Who” had not been a deliberate search for youthful demographics. “When people are trying to be cynical about modern ‘Doctor Who,’ they say, ‘Oh, they always cast these young fellows,’ ” he said. “We didn’t. It was always a young bloke who turns out to be right for it.”
Mr. Moffat said he and his colleagues quickly thought of Mr. Capaldi, for reasons he could not entirely quantify.
“He just felt incredibly right,” Mr. Moffat said. “He would just take the part in such an unexpected, different direction and overturn everybody’s preconceptions.”
At an audition at which the “Doctor Who” producers say Mr. Capaldi was the only candidate, he said he performed a test scene in which he had to ask another character to describe his new incarnation.
“The Doctor doesn’t have a mirror, so he has no idea he’s gotten older,” Mr. Capaldi said. “So he keeps asking her about his face. ‘Does it look good?’ ”
The answer he received was, “Well, it’s O.K.”
Mr. Capaldi was quickly offered the role and introduced in a live special last summer. But just as rapidly, some die-hard “Doctor Who” fans and casual viewers alike pushed back against the decision, disappointed that a role with seemingly so few boundaries had once again been given to a white male actor.
“I do think it’s well overtime to have a female Doctor Who,” Helen Mirren told the British morning show “Daybreak,” before the announcement. “I think a gay, black female Doctor Who would be best of all.”
Asked about an audience’s desire for more diversity in the lead role, Mr. Moffat said: “I just cast on instinct, really. There’s nothing against that, and we have auditioned every shape and size and type of human being for this part the last time around.”
He added that Mr. Capaldi “looks like a Doctor Who,” and could have played the character at previous ages. “He’d have been a great 20-something Doctor and a great 30-something Doctor,” he said.
Yet Mr. Capaldi’s age does not go unnoticed in his premiere.
Jenna Coleman, who plays the Doctor’s adventuring companion, Clara Oswald, said that some of their very first scenes together required her to comment on how different he looked from his predecessors.
“My lines were like: ‘But he’s so old! Why is he gray? Why has he got lines on his face?’ ” Ms. Coleman said. “I didn’t know him at all at this point. In between takes, I was like: ‘Oh, Peter, I’m so sorry. Terribly sorry. You look great.’ ”
Mr. Capaldi approached all the kidding about his age as if it were a form of hazing.
“Sometimes, I get a bit annoyed with it,” he said. “I don’t think I’m old. I’m 56. Maybe people think that’s ancient. I’m not an old man.”
Mr. Iannucci said that the humor was probably intended more for younger viewers “now getting a Doctor who’s a little bit older than they’re used to.”
On “The Thick of It,” Mr. Iannucci said, “every member of the cast had to get used to some insult made about them, usually from Peter Capaldi. It’s about time he had jokes about his own physical appearance.”
Mr. Capaldi said he had consulted “Doctor Who” forerunners like Mr. Smith. Now, he spoke about his continuing work on the series with a mixture of trepidation and the kind of certainty that only comes with seasoned experience.
“I took Matt to lunch and he came in on crutches,” he recalled, “and I said, ‘What happened to you?’ And he said, ‘This show.’ I thought: ‘My God, you’re 30 years younger than me and you’re on crutches. What’s going to happen to me?”
With his first season nearly under his belt, Mr. Capaldi did not have to look into the future to believe that he would fare just fine.
“I survived without any injuries,” he said. “It keeps you fit. It’s great to wake up in the morning and think, I’m Doctor Who.”so