Ramanujam "Ambi" Iyengar, a straightforward consumer protection advocate from Thiruvallikeni, expects everyone to follow the law and prosecutes those who violate it, but his efforts are unsuccessful due to circumstantial evidence favours the accused.

Ambi's suppressed anger manifests as an alter ego called Anniyan, a grim reaper-themed vigilante, who seeks out corrupt and indifferent individuals in order to eliminate them.

Ambi secretly loves his neighbour named Nandini, a medical student and an aspiring Carnatic singer, but never expresses his feelings due to fear of rejection.

Stating that Vikram was selected for the lead role months before the theatrical release of Pithamagan, Shankar added, "I don't look at actors that way.

[22] Expressing hope that it would be "an important movie" in her career, Sadha accepted the offer, working on the film for 120 days, adding, "I have a major role to play in Anniyan.

"[30] Comedian and character artiste Charle played an unnamed wastrel and alcoholic who extorts money from his ageing parents for his expenses and is considered by Anniyan to be a liability to society.

[43] Shankar, who grew up admiring the works of cinematographer P. C. Sreeram in films like Nayakan (1987), Agni Natchathiram (1988) and Idhayathai Thirudathe (1989), desired to collaborate and was in talks with him to handle the photography.

[66] The aradhana, held on the fifth day of the festival, witnesses exponents of carnatic music from across the world converging at his samaadhi, where they sing his pancharatna kritis in unison as an homage to the saint.

[69] Realistic sets were erected to resemble the actual venue and leading carnatic vocalists Sudha Ragunathan, Sirkazhi G. Sivachidambaram, O. S. Arun, P. Unni Krishnan and instrumentalists such as violinist A. Kanyakumari, mridangam exponent Umayalpuram K. Sivaraman were recruited to add a touch of authenticity.

As part of their roles, the male supporting actors who appear in the song were required to wear a panjakkacham and angavastram, leaving most of their body exposed to the freezing cold.

[17] In December 2004, a ten-day shoot was held in Mumbai for the item number "Kadhal Yaanai" featuring Vikram alongside a top model, whose identity was initially undisclosed to generate curiosity.

For designing the website, Shankar wanted to recreate hell and approached the Visual Computing Labs (VCL) of Tata Elxsi, a Mumbai-based company with which he had collaborated earlier for the song "Girlfriend" in Boys.

Aerial views and paintings of the city were stitched together along with computer generated images (CGI) of clouds to create the long, one-piece camera zoom.

[36] The film sheds light on the increasing social apathy and public negligence, and attempts to address these issues which plague the society and hamper the development of India.

In an interview with The Hindu, Shankar elaborated on the message he conveys through the film:[15]I have travelled to many parts of the globe, especially the developed countries, and I am fascinated by the rapid strides that they have made in all fields.

The issue that I am trying to solve is inherent weakness within our society and some motivation to move forward like other nations.Following the release of the trailer on 7 May 2005, the film was believed to be inspired by the 1886 Robert Louis Stevenson novel, Strange Case of Dr Jekyll and Mr Hyde.

[101] Sify compared the character of Ambi to the protagonist of The Mask (1994), a film about a "mild mannered guy changing into a one-man army, craving to see natural justice realised".

[59] While production was completed in March 2005, the re-recording which began in April took more time than anticipated lasting nearly 45 days with Harris Jayaraj being blamed for further delays.

"[36] Krishnakumar wrote for Rediff.com that learning from the debacle of Boys, the director went back to his strength by taking a social theme, spicing it up and serving the perfect commercial fare.

[114] Visual Dasan of Kalki wrote that both the art director Sabu Cyril and composer Harris Jayaraj lent their hand to Shankar's grand visions, adding that everyone could bow down to Shankar's social responsibility of exaggerating colorful entertainment and sensitising the fan by watching Anniyan once while appreciating Vikram for his performance in three different characters and that Anniyan's transformation reminded him of Jim Carrey from The Mask.

"[117] Nitin V Nambiar of The Times of India criticised the "poor recording" and "strictly average" dubbing, noting that Shankar's direction chooses "palatability over plausibility".

[118] Following the commercial success of the Telugu version Aparichitudu, which was a runaway hit in Andhra Pradesh, Shankar was felicitated by Sri Venkateswara Social and Cultural Organisation (SVESCO) of Tirupati and was conferred the title 'Darsaka Brahma'.

[2] The MG raised through audio rights and movie distribution to theatres in Andhra Pradesh, Kerala, Karnataka and overseas fetched a combined sum of ₹224 million.

[citation needed] With the theatrical rights and pre-release booking, the trade circuit predicted that the film would recover its cost within ten days of its release.

[137] At the completion of a 50-day run at Sathyam Cinemas, Anniyan "recorded the highest 50 days collection for a Tamil film from a single screen in the world" earning ₹11.29 million.

[citation needed] However, the film's Hindi version Aparichit opened to poor reception earning only ₹21 million across 150 screens and was declared a flop by Box Office India.

[155] In another August 2005 seminar conducted to mark the anniversary of the Quit India Movement, where participants were urged to actively fight corruption, the convener T. Hema Kumari referred to the films Bhaarateeyudu (the Telugu dubbed version of Indian) and Aparichitudu.

[162] Following the stupendous success of Aparichitudu in Andhra Pradesh, the State Transport Corporation of Warangal named a bus Remo, after the character played by Vikram in the film.

[142] When stand-up comedian and television anchor Bosskey launched a quirky play titled Dada (Don) in October 2005, he named the cast after famous characters in Tamil films.

[163] Similarly, in the 2013 comedy film Onbadhule Guru, in which the characters were named after popular protagonists of Tamil cinema, a member of the supporting cast was christened Anniyan.