Cast


 * Aamir Khan as Ranchoddas Shyamaldas Chanchad/Chhote/Phunsukh Wangdu – One of a group of three friends in the university. He went missing after graduation and his two friends traveled across India looking for him. Rancho, as a student, was intelligent and had brilliant methods to be successful and content in life. He fought against the wrong system of teaching and was often punished for this. Also he was in a romantic relationship with Pia. At the end of the film, he is shown to become a famous scientist and prospective business associate.
 * Kareena Kapoor as Pia Sahasrabuddhe – Viru "Virus" Sahasrabuddhe's youngest daughter who pursues being a doctor and Rancho's (Phunsukh Wangdu) love interest. She was engaged to another person but gets attracted to Aamir Khan's character in the film.
 * R. Madhavan as Farhan Qureshi – the film's narrator and a friend of Rancho and Raju. His father wants him to be an engineer despite his love for wildlife photography. Towards the climax, it is shown that he convinces his parents about his love for photography and embarks on the journey for the passion of wildlife photography with the help of his friends.
 * Sharman Joshi as Raju Rastogi – the third member in the gang comprising Farhan and Rancho. In his college days, he came from an impoverished family with a mother who's a retired school teacher and a paralyzed father who was a postman. His sister is unmarried in the film during his college days. In the present day, he is shown to be a settled married man in Delhi.
 * Boman Irani as Dr. Viru Sahasrabuddhe – better known as "Virus", he serves as the strict college director and Pia's father. He is the film's "antagonist". He went on to another level of acting in altering his dialogue delivery and thereby bringing a crude strictness to his character. His character focuses on a person who is highly competitive and challenging in nature and he has done complete justice to the role. towards the end he is shown to be a changed man in terms of his teaching ideologies.
 * Omi Vaidya as Chatur Ramalingam – better known as "Silencer". Omi plays a nerdy character born in Uganda and completed his schooling in Pondicherry. He believes in mindless memorizing and also sneers Rancho's ideas just like Virus. It is heavily implied that he was called Silencer because he popped "bengali baba's" pills and farts, blaming the output on others, plus cramming on exams by handling out pornographic magazines to the fellow students in order for him to rise his grades. After being insulted due to his speech, he swears retaliation and vows to be more successful. In the present part of the film, he is shown to be a very successful person holding the vice president position of a company. He mocks Rancho into signing a declaration defeat only to learn that Rancho is actually his client, Phunsukh Wangdu and later accepts the fact that Rancho is more successful than him. Vaidya was chosen by Hirani for the role mainly because of his dialogue memorization and his typical slang.
 * Rahul Kumar as ManMohan – Better known as "Millimeter" or "MM". He is a young man who does chores in the college such as helping students by ironing their clothes, finishing assignments, and getting groceries, in return for money. In the present day, he works as a faculty member in Rancho/Chhote/Phunsukh Wangdu's school in Ladakh.
 * Mona Singh as Mona Sahasrabuddhe – Pia's elder sister and Virus's elder daughter. She has a supportive role in the movie.
 * Sanjay Lafont as Suhas Tandon – a materialistic comic character who is Pia's ex-fiance.
 * Parikshit Sahni as Mr. Qureshi – Farhan's father, a strict but loving parent who just wants his son to be settled in life.
 * Amardeep Jha as Mrs. Rastogi – Raju's mother, who is a retired schoolteacher and tireless complainer.
 * Javed Jaffrey as the real Ranchhodas Shyamaldas Chanchad – a person Raju, Farhan, and Chatur meet during the Shyamaldas Chanchad funeral service. His character is shown to be a corrupt person right from childhood, taking benefits from 'Chhote' in his homework and his exams. His father sends him to London and sends 'Chhote' to ICE to gain an engineering degree in his name.
 * Arun Bali as Shamaldas Chhanchad – father of Ranchhoddas Shamaldas Chhanchad. He notices the servant boy's intelligence and arranged for him to get to college.
 * Dushyant Wagh as Centimeter/Older ManMohan – the present day Millimeter who works as Rancho's/Phunsukh Wangdu's assistant in Ladakh
 * Ali Fazal as Joy Lobo – the student who has similar passion for machines like Rancho. After Virus tells him that he would not graduate, he commits suicide.
 * Akhil Mishra as Librarian Dubey
 * Rohitash Gaud as Ranchhodas' servant
 * Achyut Potdar as Machine Class Professor
 * Madhav Vaze as Joy Lobo's father.
 * Olivier Sanjay Lafont as Suhas (Pia's fiance)