Megastar Amitabh Bachchan has just turned 64 but, with every passing year, the phenomenon called 'Big B' continues to cast his spell over millions of admirers.

If anything, his charisma is only on the rise, as much as his zest for work, with filmmakers and advertisers more than eager to capture his magnetic personality on screen, says Bollywood Trade.

So what is it about the Bachchan persona that keeps his popularity increasing year after year among his fans and filmmakers alike?

"I think it has a lot to do with the actor in him, who is getting better and better with years. It is like old wine that keeps getting better over the years," says filmmaker Ravi Chopra, who has worked with him in different stages of his career.

"The nuances which he brings about in acting today are better than ever before. I consider this phase as the zenith in his career," says Chopra, who is involved with the Big B's forthcoming, "Babul".

Chopra feels that one of the factors that keep filmmakers inspired to cast him in newer roles is Bachchan's hunger to excel and go beyond his own potential.

"This hunger to excel becomes dormant in many actors over the years. But he is still the same enthusiastic actor taking up each role with a vengeance, which makes him a great person to work with," he says.

However, the biggest attribute about him that Chopra adores, like most other filmmakers, is his humility.

"It remains the same irrespective of his ever-increasing popularity. He is never known to throw his weight around," Chopra says.

According to script-writer Javed Siddiqui, who had last written "Major Saab", the one factor about Bachchan that has not changed over the years is his ability to render a certain conviction to even the most implausible characters on screen.

"There is this amazing quality about his performance that makes even the most illogical character seems convincing on screen. Once upon a time it was evident when he played the protagonist in Manmohan Desai's films with tremendous poise and élan.

"I am sure Desai himself must have been wonder-struck about how his characters looked more believable, thanks to Bachchan. His performances continue to give the same credibility today to most other projects," Siddiqui says.

Siddiqui adds that Bachchan's increasing popularity makes his films a safe bet at the box office.

"A Bachchan starrer is more often than not assured of safe returns. Every writer, producer and director is compelled to create a project that will suit him, thanks to this factor," he reflects.

However, Siddiqui feels that he personally does not approve of Bachchan making himself available for every person and project.

"When anything is there for the asking, it loses its importance. Today, Bachchan is present in every second advertisement campaign on screen, thrice a week he is seen on KBC and his films play round the clock on the television channels. Years ago one would wait for a star's release which would happen only once in a year. Too much exposure can be killing," he says.

Director Apoorva Lakhia, who worked with Bachchan in his forthcoming release "Ek Ajnabee", is full of interesting stories about his experience of working with the enigmatic actor.

"I feel privileged to have had the opportunity of working with him. Actors like him come only once in a lifetime, and they should be allowed to do what they want to do. He has reigned over three generations of audiences, my grandmother, father and me. I guess it is every director's dream to work with him," Lakhia says.

According to him, the most inspiring factor about Bachchan is his undying enthusiasm and approach towards work. "He is an actor who exudes more energy and enthusiasm than any of us. It is simply contagious," Lakhia smiles.

Veteran distributor, Balkrishna Shroff of Shringar Cinemas, who has released some of the most notable Bachchan starrers, describes Bachchan as a once-in-a-lifetime phenomenon.

"There will never be any other superstar like him now or in the future. I don't think his charisma will ever fade. Moreover, he has a fan following of three generations, which is rare for any public figure. In fact, it is even more than the time when he was the reigning superstar during the 'Deewar' and 'Sholay' days," Shroff says.

Like Ravi Chopra, Shroff too feels that it is Bachchan's humility and simplicity that makes him doubly adorable.

"I don't ever recall any phase in his career when he was high handed or arrogant to his producers. It is his humble quality and his approachability that makes him even bigger than his achievements."