Unless it is a very convincing joke, Dan Aykroyd most definitely believes in the existence of UFOs. We get on to the subject and his eyes start doing something strange. They bulge from his head, as if to punctuate his thoughts. And they are strange thoughts. "There's this one website I go on a lot," he says, "that just ends all debate about whether they're real or not, and that's Mufon.com."

Aykroyd is the "Hollywood consultant" for Mufon (it stands for Mutual UFO Network), which seems to involve keeping abreast of developments in the UFO-sighting world and promoting the organisation. "Basically, [Mufon are] scientists from all kinds of disciplines that have formed this group to analyse what is real and what is a hoax. Now you could say every one of them is a fake - that footage of 200 whirling white dots in the sky, or the Phoenix Lights [a series of lights seen over Phoenix, Arizona, in 1997] - which 17,000 people saw - the Tinley Park sightings in Illinois, where whole suburbs saw these triangles and wedges go over at three miles an hour. Is it a mass hallucination? If so, why is it appearing on digital cameras and film? They're coming and going like taxis."

I wasn't sure about bringing up the UFOs so soon because it makes Aykroyd seem so, well, odd, and that's not the way he comes across. Or not entirely. He certainly has his eccentricities, and I don't just mean the physical oddities to which he has drawn attention in the past: he has webbed toes and eyes of different colours, one brown and one green. He wears his black motorcycle boots everywhere, even on the beach (perhaps because of the webbed toes?) and he is wearing them today, providing an edge to his suit. He always carries his police badge on him, from his time as a reserve officer for the Harahan police department in Louisiana; he pulls the badge out of his jacket pocket to show me. But he is also gentle (he is a man who can fill an armchair - it makes me want to give him a cuddle), articulate, serious even, and doesn't lack self-awareness.

Before meeting Aykroyd in the empty bar of a London hotel, I watched a programme he did last year in which he was interviewed for an hour and a half about aliens by a "ufologist" called David Sereda. At the end of it, Sereda earnestly described the actor as "one of the greatest minds in our world at this time" and Aykroyd had the grace to look slightly embarrassed. The whole thing could be a long-running joke, perhaps an attempt to keep people interested in his films Ghostbusters and Coneheads, but it's a lot of trouble to go to just to promote old movies. Does he really believe it? "I do, absolutely, and I'm not alone." He is very serious.

So why are they here? "There's that old theory that humankind and this planet were seen as the centre of the universe. That was thrown away - how could we be so presumptuous? Well, I subscribe to that. I think we are the centre of the universe and that is why they're coming. They're visiting because this is the planet that produced Picasso, the atom bomb, penicillin ... there are so many advances in science, art and culture." But if they are able to travel here, surely they are more advanced than us? "Oh, they have technology better than ours, but they didn't paint like Renoir, they don't dance like Mick Jagger, they don't write like Samuel Johnson or William Faulkner. They are envious of us. We have the most beautiful planet - the Rockies, the purple fields of the United States, the Lake District, the Pyrenees, the turquoise seas of the tropics. They don't have that. They may have gelatinous pools and crystal mountains and they've got the technology to flip from planet to planet or dimension to dimension but, you know, Keith Richards didn't come from there." I'm not so sure about that last bit. Does he think there are aliens living here, among us? "I think there are possibly some hybrids here. I think many are here for good purposes and want to improve our planet but I think some are here for malevolent purposes." Like what? "Probably extracting sperm and ova, or taking cow's lips and anuses for delicacies, being cruel to animals, that kind of thing."

Aliens and cow anuses. I could talk to Aykroyd about this for hours and so could he, probably, if he was certain I was taking him seriously. But on to more mundane pursuits. Aykroyd is in London to scout out a location for the first UK branch of the House of Blues, the music venue and restaurant chain he co-founded. His band, the Blues Brothers, still perform at the opening of every new club, even though he sold the company last year (Aykroyd has a strong entrepreneurial streak - he remains as a consultant for the House of Blues, has a winery, and imports an expensive brand of tequila into his native Canada). The other reason he is here is to promote his new film, I Now Pronounce You Chuck and Larry, in which a widowed firefighter, Larry, persuades his womanising friend Chuck (Adam Sandler) to pretend they are having a relationship and register a civil partnership so as to provide financial protection for his children. It has received terrible reviews, with critics complaing about tired, hateful "jokes" about dropping the soap in the shower, vague racism and Sandler as an unlikely sex symbol.

Aykroyd, always likable, puts in a good show as the fire captain, but is this what his career has come to? Now 55, he was influential as one of the original cast members of Saturday Night Live, had success with the Blues Brothers film and band, was nominated for an Oscar for his role in Driving Miss Daisy and wrote and starred in several hit films including Ghostbusters. But in recent years, his career hasn't looked so good: he appeared in the sequel Blues Brothers 2000, which flopped, and took small parts in films including Pearl Harbor and 50 First Dates.

I think I have irritated him. "I went as far as one could go in the business - I wrote seven movies and got them made, and starred in them and produced them. It's impossible to maintain because a new generation comes up and one has to relinquish one's position. There are new stars, new types of humour, new directors and writers and they're referencing other things, so for me to sit and home and moan that the phone isn't ringing, that's not my style. I've had 30-plus years in this business, an amazing run, and I think now it's time for the new generation to come up. The torch has been passed to Will Ferrell, Adam Sandler, and one day the time will come for them to pass the torch. I get offered things where they want a good character performance. I've actually turned down six movies in the last year because between the expansion of House of Blues, my schedule with the band, the tequila, the wine business, and raising three kids and maintaining a marriage of 25 years [he has three daughters with the actor Donna Dixon], I don't have much time to be away any more."

Aykroyd founded the Blues Brothers nearly 30 years ago with his friend John Belushi and the band is still going 25 years after the latter's death from an accidental drug overdose. Belushi, the wild one in the partnership, had been a drug addict for several years and died at the Chateau Marmont hotel in Los Angeles, after injecting heroin and cocaine. Did Aykroyd feel there was more he could have done? "I poured a lot of coke down the toilet. His wife and I hid a lot of vials or crushed them. We did everything we could but ultimately he was very wily and we're all captains of our own ship. There wasn't much we could do if he was determined to get his hands on it. He would run away or get really mad at us. This was before the time of the active intervention that you see today. We did try to get an intervention together but he bolted from the room. The last night I saw him alive, he was getting into a white limousine headed to Jersey with a coke dealer. I wasn't enough to attract him away from that." He thinks for a minute. "Of course I feel bad about it but I can't lay too much guilt on myself."

For a long time, he found Belushi's death extremely hard to deal with. "It stopped me dead. I was 29, he was 33, we had a hit record, hit movie, everything laid out before us. It sent me into a mode of abandonment of safety - I drove my motorcycle faster than I should have, I did a lot of partying, I hung out with people I shouldn't, but then ultimately I came round and realised I had something to give, I had a contribution to make, so I went on and did some of my best work after he died."

It was his parents (his father was a civil engineer, his mother a secretary) who started Aykroyd on the acting path by enrolling him at an improvisational class. They didn't particularly want him to be an actor, they just thought it would help calm their hyperactive son - he had been expelled from two schools for acting up and a psychiatrist had diagnosed mild Asperger's syndrome, a condition on the autism spectrum, because Aykroyd had a few tics and had shown signs of obsessive compulsive disorder.

His early experience would later come in useful when Aykroyd joined Second City, the improvisation comedy troupe, where his contemporaries included John Candy, Eugene Levy and Bill Murray. For a while, his mother, especially, had hoped he might become a priest: Aykroyd had been educated in a seminary for four years ("the Catholic school board had a better system of education at that time"), but they sent his parents a letter saying Aykroyd was not a suitable candidate. "They were disappointed because I had to find a new high school, but I was totally relieved. I'd make a bad preacher. I could sound good, acting it out, but who am I to tell other people how to live their lives?"

It was in Ottawa, where Aykroyd grew up, that he discovered the blues. "There was a booker who brought all the major blues artists into town. I would sneak into this club through the back door and there were all of the great players, so very early on I was being exposed to it." As a teenager, he would walk around Ottawa dressed like Charlie Musselwhite, the blues musician, wearing big shades and a long raincoat (the inspiration for his Elwood Blues character).

What I find most endearing about Aykroyd is his wide range of interests and the intensity with which he follows them: motorbikes, wine, music, politics ("I think Hillary would make an extremely capable president. She brings Bill along, she has knowledge and experience, and I see great leadership strength there"), law enforcement and criminology. His interest in the paranormal runs in the family - his grandparents held seances, and his younger brother Peter is a "psychic researcher". He says he has seen UFOs - some glowing discs travelling at high speed above his house at Martha's Vineyard in Massachusetts. Another time, he says, he was outside in the street in New York, where he lives much of the time, on his mobile phone to Britney Spears (he played her father in a terrible film called Crossroads, and she had phoned to ask him to appear on Saturday Night Live with her) when he saw a black car across the street and a very tall man get out of the back. He looked back a split second later and it had vanished. The man, he says, had given him a "dirty look" that he took as "a warning". He didn't say if he thinks the warning was against working with Spears again.

Does he mind what others think about his beliefs? "Well, there was a poll in America which said 54% believe [in UFOs], so that's a majority - I'm with millions of people. How about the hundreds of thousands who have seen them? Or the ones who have been taken up in the ships?" He really thinks people have been abducted? "Oh, absolutely, many. I've spoken with very credible witnesses who have shown me their marks."

He points abstractly to his knees. "To those who don't believe and don't want to believe and think I'm crazy: good. Let the sceptics come and tear down the sighting. If I say I saw a black wedge go over my car, let them tell me that it was a cloud. I accept sceptics, you've got to have challenges. Please provide your explanations because that will help me to realise, along with my fellow researchers and ufologists, what is real and what isn't." That is sensible enough.

It is reassuring to know that Aykroyd doesn't appear to have lost his grip on reality. Or maybe he is right. Or maybe the whole thing is a joke. I don't think so, but suddenly I'm not sure what is real and what isn't. He walks away, dressed in a dark suit and sunglasses, an older, cuddlier Blues Brother. I can't see if he's laughing.

· I Now Pronounce You Chuck and Larry is out on Friday.