Age is just a number for Amitabh Bachchan, who turns 75 on Wednesday. His work diary is already spilling into 2019, but he remains firmly "not out", continuing a glorious journey that has lasted over four decades.

In an industry where roles are seldom written for veteran actors, Bachchan is experimenting with looks, characters and themes with an enthusiasm and passion of a newcomer.

"When he comes on the set, he is always in character. I never feel like I'm directing Mr Amitabh Bachchan. I feel I am directing the character," Umesh Shukla, who is working with the "living legend" in "102 Not Out", told.

"When he steps out of his vanity van, he comes out in the type of mood that is required for a scene and makes the atmosphere like that. It's fun to work with him," added Shukla, who has cast veteran actor Rishi Kapoor as a 75-year-old and Bachchan as his 102-year-old father in the movie.

Another challenging role that Bachchan has taken up is in "Thugs of Hindostan", which will see him in an armoured look with a heavy beard. His co-star Aamir Khan, who will be sharing screen space with Bachchan for the first time, has been a long-time admirer.

"His aura, action, every nuance of his acting was such a fulfilling experience in cinema," Aamir said of watching Big B's films in theatres in his younger days.

But as he rings in his milestone birthday, Bachchan wants no celebrations.

He is off to the Maldives for a vacation with wife Jaya, son Abhishek, daughter Shweta, daughter-in-law Aishwarya and granddaughter Aaradhya. And he will be back to work soon.

"A small and peaceful family time is more than enough for me," Big B had once said.

Film historian S.M.M. Ausaja, whose forthcoming book "The Bachchans" retraces the journey of what he calls the "premier family of Bollywood", said Bachchan's "extraordinary" success is a "spectacular example of talent and luck".

"If you're very talented and low on luck, you're Naseeruddin Shah. Or if you're low on talent and high on luck, you're Jeetendra. In Mr Bachchan's case, it has been the right mix. He was unconventional in looks and amidst a string of actors for whom it was the romance genre all the way, he was like a whiff of fresh air. His mannerisms and style were his own," Ausaja told IANS.

Having started with a role as one of the seven protagonists in "Saat Hindustani", Bachchan next featured in the Rajesh Khanna-starrer "Anand" and did a few more films. But it wasn't until the 1973 movie "Zanjeer" that he came into his own.

"There was no stopping him then," Ausaja said, pointing to some of his "sensitive films" like "Ek Nazar", "Mili" and "Abhimaan", as well as hits like "Deewaar", "Sholay", "Mard", Naseeb", "Namak Haraam", "Kabhie Kabhie", "Don", "Hum" and "Shahenshah".

In 1978, he was at the top of his game with movies like "Muqaddar Ka Sikandar", "Trishul", "Don", "Kasme Vaade", "Ganga Ki Saugandh" and "Besharam" doing well.

A near-fatal injury on the sets of "Coolie" in 1982 led him to take a break from films and he ventured into politics from his native Allahabad. He contested the Lok Sabha election on the Congress ticket in 1984, but didn't continue his political journey for long.

He found himself named in the infamous Bofors scandal, but was cleared of the allegations 25 years later and remains apolitical even though he associates himself with issue-related social campaigns for the government once in a while.

Until 1988, when "Shahenshah" was released, Bachchan - son of legendary poet Harivansh Rai Bachchan - was in top form.

"Post-'Shahenshah', his fall began when directors started presenting him as larger-than-life characters in films like 'Toofan' and 'Jaadugar'. He endured a lot of flops and other problems until the start of the millennium when he came up with the television show 'Kaun Banega Crorepati' in 2000. In fact, Mr Bachchan was named 'Star of the Millennium' at the worst phase of his career.

"But then, after 'KBC', his meteoric rise began and he started playing his age on screen and reinvented himself with every project - be it 'Sarkar', 'Baghban', 'Bunty Aur Babli', 'Paa', 'Black'... His popularity is a binding factor for the country," Ausaja said.

For someone who was considered too thin and too tall to fit in the Hindi film industry, Bachchan's run in showbiz - to many - is what dreams are made of.

He failed when he launched Amitabh Bachchan Corporation Ltd in the 1990s, but his acting acumen has taken him places.

Bachchan has earned epithets like "Angry Young Man" and "Shahenshah", and has won the love of fans who throng the gates of his house in Mumbai every Sunday to meet him.

Shukla said Bachchan must be still working 14 to 15 hours a day. "It's commendable at his age. He is always on his toes and I have never seen him tired..."

"Sometimes we have to tell him to take it easy. He says, 'No, I'll do it'. That's his spirit," Shukla said.