After taking a one-year sabbatical from work, Raja began working on his first original script in 2012, taking help from the writer duo Subha, which eventually became Thani Oruvan.
Mohan Raja spent one year in researching the roles of Mithran and Siddharth, and another 9 months for writing the script.Raja apparently met several IPS officers and visited police training colleges for getting to know their functioning. He also took the help of biotechnologists for the illegal medical practices part.He had later said in an interview that he was excited by the project's logline - 'a cop who goes all out to hunt evil in society, and a villain, who goes all out to for money and fame'Raja reportedly turned down several remake offers, including the opportunity to direct the Ramana Hindi remake with Akshay Kumar, for working on Thani Oruvan's preproduction.Vijay was Raja's original choice for playing Mithran IPS. In early 2013, Raja refuted rumours that he's directing Vijay in Ek Tha Tiger remake, stating that he had an original script for the star.Raja then thought of making Thani Oruvan in Telugu with Prabhas, and even approached the actor with the script.However, Vijay apparently wanted Ravi to do the role of Mithran as he thought It would give Ravi his much needed breakthrough.Raja had said that casting Ravi worked to his advantage, as he 'needed someone who would trust him blindly and not feel insecure about the villain overshadowing him' for the role.Raja had billed Thani Oruvan's script as 'experimental, yet mainstream', stressing that he had given his heart and soul for making it stay that way.Raja had said that Nayanthara was his only choice for the character of Mahima, as he 'wanted an actress, who was equal parts spunky and matured, and could look & act the part of a forensic specialist'Raja would later go on to say in 2017 that he wants to do a female-centric film with Nayanthara as the lead.Arjun Rampal, Arya, R.Madhavan, Arvind Swamy, Arjun and Sudeep were all considered for the role of Siddharth Abhimanyu.Arya and Madhavan were approached for the role, but talks ultimately fell through.Finally, Raja cast Arvind Swamy as Siddharth Abhimanyu as he felt 'the actor's charisma would act as a distraction from his malevolence.'Model and Bollywood actor Mugdha Godse made her Tamil debut, as Siddharth's love interest, in Thani OruvanJayam Ravi and Nayanthara shared screen-space for the first time in their careers.The film was shot over a period of one and half years, with several production delays.The scenes involving the lead pair's training were shot in a police training camp in BangaloreAll the scenes involving police training and the romantic sequences between Ravi and Nayanthara were completed, when Arvind Swami came into the picture.Yuvan Shankar Raja was first signed as the music director, but was later replaced by Hiphop Tamizha, as Yuvan felt there won't be scope for his music in the script.The film was shot on film roll (analogue form of filmmaking) by Ramji, who claimed that it might be one of the last Tamil films to get shot in film, as the industry has moved to digital filming.The film's budget, which was initially planned to be 15 crores, surpassed 22 crores due to budget overrun.Raja and Ravi gave up their entire salaries in order 'to complete the film in the exact way that it was visualised, without a single compromise'.Sun Pictures, which had agreed to distribute the film, pulled out during the film's production, as they thought it was low on entertainment quotient.Mohan Raja had decided to have only one song, Kadhal Cricket, in the film. He later added the Kannala Kannala track, half-heartedly, in the second halfKharesma Ravichandran, a then-14-year-old British singer, who had sung the initial English bits in Yaen Ennai song in Yennai Arindhal, was roped in for Kadhal Cricket, making it her first Tamil song.The 'Theemai Dhaan Vellum' song was sung by Arvind Swamy, along with HipHop Aadhi. The song features only in the background in the scenes involving Siddharth Abhimanyu.The film was remade in Telugu (Dhruva) with Ram Charan and Rakul Preet Singh as the leads, with Arvind Swamy reprising his role.Thani Oruvan is expected to be remade in Hindi as well. Salman Khan is said to have expressed interest in the cop's role, while Arvind Swami might again reprise his villain role.Raja is currently working on a sequel to Thani Oruvan, which is said to feature Ravi and Nayanthara as Mithran and Mahima, again.