The gait is a little slower, the voice a little quieter, and the hearing aid impossible to miss. But there is no mistaking that sharply etched jawline and chin that juts so far out it looks as if it is off on its own exploration: this is, most definitely, Christopher Lloyd, sipping a glass of water in a restaurant in a central London hotel on an overcast afternoon. Though he looks about as bemused at this turn of events as – oh, let’s say – Marty McFly rocking up in 1955.

The reason Lloyd is here is because he’s promoting his latest film, I Am Not a Serial Killer, an intriguing indie horror-drama that answers the question of what a movie about killer aliens would look like if it was directed by Richard Ayoade. At least, Lloyd thinks that is why he is here: he can’t quite remember the name of his character – “Horley? Harley?” he wonders, not looking wildly bothered (it’s Crowley). While he was very happy to appear in the movie as the neighbour with a secret, the truth is, he says, with an easy-going chuckle: “Pretty much anytime an offer comes in, I’ll do it.”

Lloyd in I Am Not A Serial Killer

This indiscriminate approach to his career partly explains how he has made more than 80 movies over the course of his career, including classics such as Clue, Who Framed Roger Rabbit?, The Addams Family and, of course, Back to the Future. But really, it makes no sense at all. How can a man so blessed with talent, a character actor who can also do the big-budget cartoon roles, celebrated for playing, on the one hand, Klingon Commander Kruge in Star Trek III: The Search for Spock and, on the other, Willy Loman in The Death of a Salesman, value himself so little that he will sign up for any old script that gets posted through his door?

“Oh, I don’t know. I like being busy. What else should I be doing?” asks the 78-year-old, genuinely puzzled by my puzzlement.

You could be hanging out with friends back home, I say.

“What friends?” he says, and I laugh, but he doesn’t, eyeballs bugging out, Emmett Brown-style.

This exchange sounds sad, but it doesn’t feel so in person. It feels – like a lot of our time together – a little off-kilter and gently ironic. Lloyd turns out to be a rather endearing combination of humility and loopiness, someone who will express, in one sentence, sincere emotions and, in the next, reveal a prankster who delights in conversational awkwardness. He got his first movie break in 1975’s One Flew Over the Cuckoo’s Nest, and his breakout role was the stoner Jim on the legendary late-70s and early-80s sitcom Taxi. The connecting feature between the two was Danny DeVito, who may or may not have recommended Lloyd for Taxi (Lloyd is not entirely clear on this). When I interviewed DeVito a few years ago he spoke with warmth about Lloyd. He recounted nights when he would have dinner with Lloyd and the surrealist comedian Andy Kaufman, “and I’d look over at Chris and Andy, and they’d just be on the sofa squished up together, laughing silently at God only knows what”.

At the time, it was hard for me to picture Lloyd, who has always had a reputation as a gentle soul, with Kaufman. But on meeting him, the two of them make more sense and they were, Lloyd says, voice dropping just a little, “good friends”. When I tell him that his performance as Judge Doom, the murderous lawmaker who kills innocent cartoon characters in 1988’s Who Framed Roger Rabbit?, gave me nightmares for a year, he instantly morphs into Doom himself, slamming his hand down on the table. “GOOD!” he shouts with such force that the whole hotel restaurant goes momentarily silent while Lloyd stares fiercely into my eyes.

Lloyd as Judge Doom in Who Framed Roger Rabbit?, 1988. Photograph: Allstar/WARNER BROS.

Did he worry about giving such a terrifying performance in a kids’ movie?

“I remember I saw Disney films like Bambi and Snow White when they first came out, and I was probably about your age when you saw Roger Rabbit. And there were moments that were terrifying in those movies! So when you say how I scared you, I think –” he pauses again, and Doom returns – “PAYBACK!”

When we meet it is still a few weeks before the US election and Lloyd has recently re-watched Back to the Future 2. Last year, BTTF’s writer, Bob Gale, confirmed that he and the director, Robert Zemeckis had, as fans long suspected, based the depiction of the wealthy and – in that particular film – politically powerful bully, Biff Tannen, on one Donald J Trump.

“I was stunned by how accurate [that storyline] was. I mean, it’s just incredible,” says Lloyd, those crinkled eyes widening in astonishment at the memory. “[Zemeckis and Gale] must have been so prescient to see it. They were certainly more prescient than me.”

So how does it feel to see Biff running for the White House for real?

“Ach!” cries Lloyd, reeling back in his seat as if in physical pain. “He’s a BEAST. He needs to be put in a cage.”

If ever a time was ripe for the return of a fictional character, that time is now and that character is Biff. However, Thomas Wilson, the actor behind the bully, has never seemed to enjoy his association with the film – certainly not as much as Lloyd, who, after some initial wariness (“I had a bit of an attitude”), now happily rocks up to Comic Con events and chats to fans for hours. There haven’t been many days since 1985, when the first film was released, in which Lloyd hasn’t been asked about his career-defining role, “and I never get tired of that. Never. People tell me they decided to become scientists, or doctors, or get involved with computers [because of his character, Doc]. I remember kids seeing it when it came out and they became parents and took their kids, and it’s still going on, the same cycle. It’s just – it’s just …” he struggles for a moment to put his feelings into words. “It’s just amazing to see how it’s had such a positive impact. I never would have guessed.”

‘I was stunned by how accurate the storyline was’ … Thomas Wilson as Biff Tannen in Back to the Future 2, 1989. Photograph: Allstar/UNIVERSAL

Wilson, by contrast, has generally stayed away from the cast reunions, and his most public comment on the movie was his song, the Question Song, in which he expresses his boredom with people asking him about the films (“They shake my hand but never ask me my name/And they start asking questions that are always the same/Stop asking me the questions!”

“Yeah, Tom doesn’t really involve himself in the Back to the Future thing – he’s very distant. But now [because of Trump] he must get asked about it all the time!” Lloyd then bursts into a laugh so dry it sounds like the rustling of leaves as he imagines Wilson plagued for ever by questions about the president-elect.

Does he have a favourite Back to the Future movie?

“The third one,” he replies immediately, referring to the final film set in the wild west, which is literally no one’s favourite film in the franchise – except, it turns out, one of its stars. I must look a little surprised because he eagerly elaborates.

“I get to have a romance in that movie,” he explains, with no small amount of satisfaction.

Aha.

Lloyd was born in Connecticut, the youngest of seven children in a well-to-do family, and worked for years in the theatre before he was cast in One Flew Over the Cuckoo’s Nest, at the age of 33.

“I had been up for so many movie auditions and – not even a glimmer. I just didn’t resonate. I’d be sitting there and I could feel the room,” and he waggles his fingers, as if they were specks of dust flying away. “I’d think, this isn’t working, this isn’t happening. But then it happened.”

Although Lloyd is best known for his larger-than-life roles, all eyes-a-popping zaniness, my favourite performance of his is in the comparatively little seen Things to Do in Denver When You’re Dead, as the unfortunate Olden. The scene in which he tells his old friend Jimmy (Andy Garcia) that he isn’t going to run away from what is ostensibly a death sentence is a masterclass in wry, heartbreaking understatement.

‘People tell me they decided to become scientists or doctors because of Doc’ … Lloyd in Back to the Future. Photograph: Allstar/UNIVERSAL

“I loved that movie, and I think it was a little overlooked,” agrees Lloyd. “It’s an odd one, but I love it. Andy Garcia was so wonderful to work with – he just pulls it right out of you. A special movie.”

Despite Lloyd’s success, he has never been a leading man. Instead, his greatest roles have been in films that are notably ensemble pieces, especially Taxi, Clue, Back to the Future and The Addams Family, and it is obvious that’s how he likes it.

Back to the Future always seemed like a buddy movie to me, I say.

“Yeah!” he agrees happily. “Yeah, yeah. Michael and I just had that chemistry. And he’s so wonderful, you know. What he’s had to deal with since [with Parkinson’s disease], and he just moves ahead with humour and sensitivity. I was watching Back to the Future recently – the first one – and I thought, wow, the way he moved.” Lloyd looks downwards, his voice dropping a little again.

And it was his light movement that made Zemeckis fire Eric Stoltz, who was originally cast as Marty, and hire Fox, I say.

Reverend Jim Ignatowski in Taxi, 1978. Photograph: Allstar/PARAMOUNT

“Right, right. But it was worrying at the time because we’d already done two weeks of shooting, I think. They gathered us all together at 1am, because we were doing night shoots then, and [the movie’s producer] Steven Spielberg was there, so I thought: ‘What’s going on?’ And when they told us, I thought: ‘Oh my God, I’m going to have to [establish Doc and Marty’s relationship] all over again.’ But with Michael, the chemistry was just there.”

Does he think he likes working in ensembles because he was one of seven?

“Maybe, I’ve never really thought about it like that. Who knows?” he shrugs, not overly interested in self-analysis.

I tell him a friend of mine was one of seven and it made her want to be single for ever, so as to have her own space at last.

“Hmmm, yeah, that wasn’t the case for me,” he says.

When we meet, Lloyd is two weeks away from marrying his fifth wife, Lisa, a real estate agent, whom he met at a film festival a decade ago. “We saw each other and since then it’s been … nice!” he grins.

Yet their wedding will have to wait a few more weeks. First, he has to fly to Berlin for another Comic Con. “I was sitting down at one recently, and I look around and there’s William Shatner to my right and I thought: ‘Why am I here? Where’s Michael?’ And then I thought: ‘Oh right! I’m here for Star Trek 3!’” And once again, he looks a little confused, but perfectly happy about it.

I Am Not a Serial Killer is released in the UK on 9 December