As the voice of Iron Man's computer programme Jarvis, Paul Bettany has spent a lot time hidden away in a sound booth, only meeting up with his Marvel Comics co-stars on the red carpet.

For the new Avengers: Age Of Ultron movie, however, he finally made it onto set.

But that, he says, was "a double-edged sword".

"I used to turn up for two hours in a dark sound booth with a microphone, and leave with a bunch of cash, which was great," jokes the actor. "Now I actually have to turn up and earn my living. The upside is, you're working with really creative, funny, interesting people, and we had a lot of fun on set."

Indeed - so much fun that director Joss Whedon was forced to reprimand the cast. "I can't speak for Joss, but I imagine it was very tricky," says Bettany. "There were a lot of characters to cover and to try and give them their moment - he did an amazing job.

"But for us, the set was very funny. They're a bunch of very creative, entertaining people who are a giggle to be around."

Apparently, Robert Downey Jr, who plays Tony Stark, aka Iron Man, wasn't even the worst culprit (that was Scarlett Johansson and Mark Ruffalo).

In fact, Downey Jr, says Bettany, who's worked with him since 2008's Iron Man, is "incredibly professional, very prepared, very witty and sharp as a tack".

In this sequel to 2012's Avengers Assemble, Stark tries to jump-start a dormant peacekeeping programme - but things go awry and the heroes, including Black Widow (Johansson), Captain America (Chris Evans), The Incredible Hulk (Ruffalo), Hawkeye (Jeremy Renner) and Thor (Chris Hemsworth), have to reassemble to stop the technological villain Ultron from fulfilling his ambition of exterminating the human race.

The turn of events also sees Jarvis transform into a new character called Vision, an artificial life form and "totally naive creature born as an adult, who is super smart and yet he still has the childlike features of experiencing everything fresh", London-born Bettany explains.

As befitting an Avenger, his new guise is impressive, and the 43-year-old reveals it involved six months of make-up tests "to figure it out".

"My daughter [Agnes] loved it. She's three and was very upset when I took it off at the end of the day. She wanted 'Purple Daddy' back."

Bettany, who also has son Stellan with his wife Jennifer Connelly and is stepfather to her son Kai, says he found himself in some strange situations while attempting to keep his character's get-up under wraps.

"Once you're in the costume, you're travelling everywhere on set in this cloak that's sealed up. You couldn't see anything, I didn't know where I was, but people try to take photos," he explains.

Of course, this being an Avengers movie, Bettany was required to do stunt work - not that any of the choreographed fight sequences fazed him. "The stunts were so great on this and we were so well prepared. I remember in the old days, it was a little hairy," he says, laughing.

"On one movie, I got dropped about 25ft on a rig and it didn't slow down. It was just dropped and it was really painful. But now, they really don't want to damage [the likes of] Chris Hemsworth. That's a costly thing to damage."

After studying at Drama Centre London, Bettany regularly appeared on stage before making his TV debut in 1994, in an episode of detective series Wycliffe.

In 1997, he was cast in a small part in the holocaust film Bent, but it would be another three years before he landed his first leading film role, in 2000's Gangster No. 1.

This led to Brian Helgeland, the writer and director of A Knight's Tale, writing the part of Chaucer in the 2001 movie specifically with Bettany in mind. Helgeland then spread the word about the actor's talents to his friends, including Ron Howard, who cast him in A Beautiful Mind with Russell Crowe.

It was also through that Oscar-winning movie that he met Connelly. The pair worked together again on 2009's Creation, and most recently in Bettany's directorial debut Shelter, a story about two homeless people, which is set for release later this year.

"I've always wanted to [direct]. I like telling stories and I wanted to try to do it in a different way. I'm not in the movie. I was, and then I fired myself," he says, grinning.

Though he enjoyed the experience, he admits the shoot in New York City, where he lives with his family, was "an intense period of time", particularly as the parents were both working on the same project,

"Work came home with us," he admits. "It was really good for the kids that the production only went on [for] 20 days.

"Usually we tag-team; there's always one of us at home, but on this, we were having to both work. And this was really intense in the subject matter, and some of the things that happened in the scenes were really dark. I'm very glad for my children that it's ended."

The experience hasn't put him off directing more in the future, though.

"Directing taught me more about acting than 20 years of acting," says Bettany.

That said, he still enjoys letting somebody else take charge.

"It's lovely to totally let go."

Avengers: Age Of Ultron is in cinemas now