Yashvardhan "Yash" Raichand (Amitabh Bachchan) is a rich business tycoon, living in Delhi with his wife Nandini (Jaya Bachchan) and two sons Rahul and Rohan. Rahul is the elder son and was adopted by Yash and Nandini at birth. This is known to everyone in the Raichand household, except Rohan. The Raichand household is highly patriarchal and follows traditions. While grown up, Rahul (Shahrukh Khan) comes across the vivacious Anjali Sharma (Kajol), and they eventually fall in love, but their love is forbidden, because Anjali is from a low-income background. Yash and Nandini soon send Rohan (Kavish Majumdar) to a boarding school, which all the males in their family have attended. Yash announces his desire for Rahul to marry Naina (Rani Mukherji), as he believes that parents have the right to choose their offspring's spouse, but Naina learns that Rahul is in love with Anjali, and encourages him to pursue her. When Yash comes to know of this, he is enraged, and Rahul promises not to marry Anjali, as he wants to please his father. En route to tell her of his promise, he discovers that her father (Alok Nath) has died, and decides to marry her despite his father's hostility. Yash learns of the marriage and disowns Rahul, whereupon Nandini and Rahul share a tearful goodbye. Nandini sends Sayeeda (Farida Jalal) (Rahul and Rohan's nanny) to watch over him, so that he does not feel separated from a mother's love. Rahul visits Rohan in the Boarding School and begs of Rohan never to ask where he went or why he left, and asks that he take care of Nandini.

Ten years later, Rohan (Hrithik Roshan) returns home from boarding school and accidentally overhears a conversation from his grandmothers (Achala Sachdev and Sushma Seth) concerned Rahul's adoption and the reason why Rahul left home, whereupon Rohan vows to reunite the family. He learns that Rahul, Anjali, and her younger sister Pooja (with whom he shared a playful relationship during his childhood) moved to London, and tells his parents that he wishes to pursue further studies in London. Yash is skeptical at first, but once Rohan emphasizes that he is following the family traditions, his parents agree to let him go. In London, Rahul and Anjali now have their own son, Krish (Jibraan Khan). Pooja (Kareena Kapoor), now a popular fashion-obsessed diva, is a student at King's College London, where Rohan enrolls. He and Pooja meet again and he tells her that he has come to London to bring his brother and sister-in-law back home. Pooja tells Rahul that Rohan is her friend's brother who currently has nowhere to stay. After some persuasion, Rahul agrees to let Rohan stay as Rohan uses the name 'Yash' to hide his real identity. Right before the family attends Krish's school function, Sayeeda and Anjali find out his real identity, though they promise to keep quiet. At Krish's school function, Krish leads his class in singing Jana Gana Mana (Indian national anthem), and later recites advice which Rohan had given him. Rahul, having given Rohan this advice ten years ago, now realizes that Rohan is his brother. Also at this point, Rohan and Pooja fall in love.

Rohan begs Rahul to come home, but Rahul refuses. Pooja convinces Rohan to invite his parents to London; Rahul and Nandini are overjoyed to see one another, but Rahul still refuses to talk to his father. When Yash goes to a store, Anjali stumbles by and Rohan reveals that they have been living in London the whole time. Yash soon learns that his mother had died and that her last wish was for Yash, Rahul, and Rohan to light her funeral pyre together. Therefore, the entire family attends the funeral, and Nandini tells Yash that she thought he was wrong for disowning Rahul. Rohan and Pooja convince Rahul to talk to Yash, who apologizes and allows Rahul and Anjali into the house. Rohan and Pooja are married, and the family hold a belated celebration of Rahul and Anjali's wedding.

• Amitabh Bachchan as Yashvardhan "Yash" Raichand, a Delhi based business tycoon. Fiercely dominating, he insists on controlling the life of his wife and sons. Amitabh Bachchan was Karan's first choice for the portrayal of Yash Raichand. Karan added, "As I wrote the film, I realised that Yashvardhan Raichand is the backbone of the film and I could only see one actor playing the role — Amitabh Bachchan." Bachchan, on his part, agreed to do the film without a script narration. Karan mentioned that he was initially scared to direct a star of the stature of Bachchan, but the latter "soon became an actor instead of a superstar".
• Jaya Bachchan as Nandini Raichand, Yash's wife. She shares a close bond with her sons but remains in the shadow of her husband. According to Karan Johar, Jaya Bachchan was the "obvious" choice for the character of Nandini, and added that her "acting prowess and stature" were the other reasons for him preferring to cast her. The film also marked the return of Amitabh and Jaya Bachchan together on screen after a gap of 18 years. Of her character, she said that Nandini was an extension of her own self. She elaborated, "My personal feelings towards Shah Rukh are similar to what I was supposed to portray in the film. There's something about him that makes me want to mother him." She added that she modelled her character on Karan's mother, Hiroo Johar, who "is a very emotional and sentimental person."
• Shah Rukh Khan as Rahul Raichand, Yash and Nandini's adopted son. Rahul feels indebted to his parents and tries to fulfill all their wishes. However, he falls in love with Anjali and invites the wrath of Yash. When Karan offered the role to Shah Rukh, he immediately agreed to do it and accommodated his dates, despite having several other commitments. Khan described the character of Rahul by saying, "I love the vulnerability and the honesty in his eyes. He has the appeal of a boy next door. Besides, his intensity and ability to convey emotions without words is amazing."
• Kajol as Anjali Sharma, Rahul's love-interest and later, wife. She is a fun-loving woman living in the Chandni Chowk area of Delhi. Belonging to a lower socio-economic group than the Raichand family, she is not accepted by Yash as his daughter-in-law. Karan was initially hesitant to cast Kajol in the film, as he felt that she would refuse the offer due to her recent marriage. Kajol, however, was moved to tears during the script narration and agreed to do the film. In an interview to Filmfare, Kajol said, "One tiny fact that Karan forgot to mention during his narration was just how much Punjabi my character spoke in the film. I nearly died when I saw the lines of dialogue on the first day of shooting." However, she learnt the right pronunciation and diction with the help of Yash Johar and the crew members.
• Hrithik Roshan as Rohan Raichand, Yash and Nandini's biological son. He wants his elder brother Rahul to return home. Karan signed Roshan to play the character of Rohan after watching a rough cut of the latter's debut film, Kaho Naa... Pyaar Hai (2000). Roshan described his character as a "buffer" in a film that primarily focused on Amitabh Bachchan and Shah Rukh Khan.
• Kareena Kapoor as Pooja "Poo" Sharma, Anjali's younger sister. She is a sophisticated girl, who helps Rohan in his plan to reunite Rahul with his parents. After spotting Kapoor at a party organised by Bombay Times, Karan decided to cast her immediately for the role of the glamorous diva, Poo. Kapoor stated that in her opinion, Kabhi Khushi Kabhie Gham... was primarily Kajol's film, and that her own character was a supporting one. In order to prepare for her role, she worked hard on her dancing skills, as she did not want audiences to know that she could not dance well.
• Rani Mukerji as Naina Kapoor, a socialite girl who loves Rahul and wants to marry him, and Yashvardhan approves of her. Following the success of Karan's previous film, Kuch Kuch Hota Hai (1998), which starred Khan, Kajol and Mukerji, he wanted her presence in this film too. He thus cast Mukerji in a guest appearance. Initially, Karan wanted her presence in the film to be a surprise, but an accidental slip by Sony Music during the promotional activities led to her discovery.
• Farida Jalal as Sayeeda / Daijan (DJ), Rahul and Rohan's nanny
• Simone Singh as Rukhsaar, Sayeeda's daughter and Anjali's best friend
• Alok Nath as Bauji Sharma, Anjali and Pooja's father
• Kavish Majumdar as the younger Rohan Raichand
• Malvika Raj as the younger Pooja Sharma
• Jugal Hansraj as Rohan's friend (special appearance)

Achala Sachdev and Sushma Seth were cast as Yash and Nandini's mothers, respectively. The film also featured Johnny Lever as Haldiram (a shopkeeper in Chandni Chowk), Himani Shivpuri as Haldiram's wife, Jibraan Khan as Krish Raichand (Rahul and Anjali's son), Amar Talwar as Mr. Kapoor (Yash's friend and Naina's father). Ramona Sunavala, Jeroo Writer and Vikas Sethi feature as Poo's friends Sonya, Tanya and Robbie, respectively. Additionally, Ashutosh Singh features as Ashfaque, Rukshaar's husband. Shilpa Mehta, Shashikala and Parzan Dastur were cast as Ashfaque's mother, grandmother and nephew, respectively. Shashikala played the in-law to Rukhsaar. Punit Malhotra and Johnny Lever's real-life son Jesse Lever had small parts.

Development

After the success of Karan's debut film, Kuch Kuch Hota Hai (1998), he began work on a story dealing with the concept of "generations". The idea initially revolved around two daughters-in-law. However, on the advice of filmmaker Aditya Chopra, who thought that the male characters would be too weak, Karan decided to tweak the story-line to make it the story of two brothers.

The inspiration behind Kabhi Khushi Kabhie Gham... came from Yash Chopra's Kabhi Kabhie (1976). On being inspired by the classic, Karan quoted, "What appealed to me was the fact that the love story stretched out across generations. It began with youth and went on as the people grew older. You could say that Kabhi Kabhie is the starting point for my new film, that I am inspired by it. But the film, I am sure, will be very different. It will look different, feel different." Similarly, Karan added an extra "e" to the second Kabhi in the title of his film, due to numerological reasons. In an interview with The Times of India, Karan dispelled compar