Members of the cast in Sivaji

After the official announcement of the production company, the director and the lead actor, M. S. Guhan and Shankar began to select other members of the cast and the crew to be a part of the Sivaji team, Rajinikanth assisted in helping choose the other members. For his crew, Shankar picked his preferred technicians; with Sujatha as the dialogue writer, K. V. Anand as the cinematographer, Peter Hein as the stunt master, Thotta Tharani as the art director, Vairamuthu as the chief lyricist, Manish Malhotra as the costume designer, Prabhu Deva as the choreographer, Anthony as the editor and J. D. Jerry of Pavithra fame, Balaji Sakthivel of Kaadhal fame, Hosimin of February 14 fame and Gandhi Krishna of Chellamae fame as the team of assistant directors. Also, despite receiving rave reviews for the music composing in Anniyan, Harris Jayaraj was dropped as the music director and Shankar's orthodox music composer, A. R. Rahman was chosen, creating great expectations for the project.
The role of the comedian was tipped to go Vadivelu,[16] after he received rave reviews for his comic chemistry in Chandramukhi with Rajinikanth, but however, despite the reviews, Shankar's regular comedian, Vivek was selected therefore making it his third Shankar film in succession. Other reports claimed that popular Tamil actor, Srikanth[17] was to act as Rajinikanth's son-in-law in the project but soon the story was denied. The leading lady of the film's selection took longer than it was expected. Aishwarya Rai,[18] who had been in contention to act in the last four movies of Rajinikanth, was finally expected to be selected as she had owed a favour in return to Shankar, who helped her deliver her first hit, as of Jeans. However she rejected the offer at the time citing a lack of dates and the clashing of her other films schedules, Guru, Provoked, Umrao Jaan and Dhoom 2. Another Bollywood actress, Ayesha Takia,[19] was also considered and approved by Rajinikanth, only to reject the film claiming she had problems with the Tamil language. Soon, another Bollywood actress, was approached for the film, Rani Mukherjee[20] but Shankar decided to reject her as she had asked for a huge amount of money for her salary. Other famous actresses such as Jyothika Saravanan,[21] who had acted as the female antagonist in Chandramukhi, Trisha Krishnan,[22][23] Nayanthara[24] and Sneha[25] were considered but then rejected for the role of the heroine. In late 2005, Shankar and Rajinikanth announced officially that Shriya Saran had been selected to essay the role as Rajinikanth's love interest.[26][27]
Shriya Saran was relatively new to Tamil cinema when she was selected for the role. Despite acting in over a dozen films in the Telugu language opposite big names such as Chiranjeevi, Mahesh Babu, Venkatesh and Akkineni Nagarjuna, she had only acted in two Tamil language films which were Ennaku 20 Unnaku 18, in which she played a supporting role and Mazhai opposite Jayam Ravi. Surprisingly, several television artistes were given other roles in the film. Shankar and his crew opted for Manivannan,[28] M. S. Baskar, Livingston,[29] Vadivukarasi,[30] Pyramid Natarajan,[31] Uma Padmanabhan, Pattimanram Raja,[32] Vannakam Thamizhuzhagam Revathi,[33] Solomon Pappayya,[34] Muthukaalai,[35][36] Nellai Siva,[37] Bose Venkat[38] and M'Sia Prabhu to play roles. In February 2006, Kumudam magazine came out with a cast list, which included mimicry artists, Cochin Haneefa, Chinni Jayanth[39] and Mayilswamy[40] as well as a list of guest appearances by crew members, Kannal Kannan, K. V. Anand,[41] Thotta Tharani[42] and Shankar.[43] However, the role of the antagonist also took a while to confirm with Sathyaraj,[44] Mohanlal,[45] Sanjay Dutt[46] and Nana Patekar[47] being asked only to reject the offer citing a variety of causes. Finally news appeared that yesteryear actor, Suman,[48][49] who had appeared with Rajinikanth as his brother in the 1980s film, Thee had been chosen.
Another key role of a lawyer played by a guest actor was originally given to Mohanlal, who opted out of the role soon. The offer was then given to Amitabh Bachchan,[50] who demanded the shooting to be held within two days in Mumbai, so that he could avoid clashes of dates.[51] However, the unit couldn't respond to Bachchan's request and his role was cancelled, to be replaced by Raghuvaran,[52] who had also played key roles in other Rajinikanth starrers. However other reports claim that Amitabh Bachchan is still in the cast,[53] while so is Malayalam actor, Mammooty.[54] Another guest role was supposed to feature an actress in the form of an item number for the song, Balleilakka, a range of heroines were considered for the role including Sadha,[55] Shilpa Shetty, Manisha Koirala, Bipasha Basu,[56] Asin Thottumkal[57] and Nayanthara,[58] however early reports claimed that Mallika Sherawat had already bagged the role and completed shooting,[59] but in September 2006, Shankar said the song was yet to be finished, with Asin Thottumkal and Nayanthara in contention for the role. Due to unknown reasons, Asin Thottumkal dropped out of contention and Nayanthara paired with Rajinikanth for the second successive time after their pairing in Chandramukhi.[60] But still like the lawyer role, reports still claim Mallika Sherawat is still a part of the magnum opus project.[61]