Thenali is a Tamil remake of the American film "What About Bob?", starring Bill Murray.
The movie became a reality only because of Kamal Hassan's mega project Marudhanayagam got delayed indefinitely.
AR Rahman didn't have the dates to take up the project. So it was given to S. Bhattacharya.
In the scene where Delhi Ganesh and Ramesh Khanna Jump into the lake in Kodaikanal, they came out of the water freezing. Kamal Hassan himself prepared and gave them "Kashayam" made out of milagu and jeeragam.
The famous close up scene of Kamal Hassan's eyes were actually shot in Kamal Hassan's home while he was preparing for Aalavandhan.