1. HIndi film "Dunia Naa Maane " (1937) - Kunku in Marathi. produced by Prabhat films under the direction of V Shantaram remain a classic. The film had no background music. Only natural sounds. And for the songs also - the singing stars htting cups together, beating on the floor with stick etc to give orchestral effect to the song. And for two songs, the heroine playing a gramaphone record of instrumental music as background for the singing. A Great film, I believe it is in this film that a full length English song was sung (by Shanta Apte.
A poem by Longfellow - Psalm of life.
"In the worlds great field of battle....... as I remember).
In Malayalam film "Kodiyettam" also, I understand only natural sounds are there. No orchestral background music for the film.
2. In Padunna Puzha, the comedy King of yesteryears, Adoor Bhasi acted in the role of Mother, daughter and son (I have not seen this film, read so somewhere which I noted down)
3. The genius writer, director of Malayalam cinema Padmarajan - he dubbed for the Phayalvaan in the film "Oridathoru Phayalvaan". He lent his voice to Prathap Pothen also in the film "Novembarinte Nashtam"
4. This is my observation, and not mentioned anywhere, I have not come across at least so far. In the Malayalam film "Aana Valarthiya Vaanambaadi", I doubt - It was Adoor Pankajam who lent her voice to the Tamil actress C K Saraswathi (acting as Shanti's mother) and Aranmula Ponnamma that lent voice to S D Subbalakshmi (Hero Sreeram's mother. SD Subbalakshmi is the stepmother of dancer Padma Subramaniam, )
5. Jyothi Venkatachalam, who was Governor of Kerala acted in the Malayalam film in a guest role - "Bharya Oru Manthri"
6. Film stars act as original film stars in films. Mammootty, Mohanlal etc have acted in some films, for example Mammootty in the film "No.20 Madras Mail" as film star Mammooty.
I think, it was Satyan who acted in such a role for the first time in Malayalam. In Chathurangam he acted as Satyan, who comes for a college day celebration in the film.