I have observed Telgus can read Kannada and vice versa. but, Malayalam script although looks similar is different. however, some words can be readable by Malayalis. learning tamil alphabet is easy(acc. to me). they have no complex alphabets and special symbols etc as we have in Malayalam.
Malayalam and script of Tulu language seems much closer . while, Tamil is more intelligible perhaps than Tulu. if a Tamil speaks to Malyali he may understand and vice versa - not always true.
Tamil is so much different - they have no separate alphabets for separate sounds like we have in other Dravidian/Indo-aryan languages.
Tamil: ka,cha,tha,ta,pa.. not ka,kha,ga,kkha like Kannada or Malaylam have. also, the word formation is also very different:
Prayana(m) is a word for travel in many south Indian languages. while in tamil, they simplicized it into pa-ya-nam.
common example is: you write cricket - as cricket in Kannada or Telgu or Malyalam. in Tamil you write: Ki-ri-ke-t .. name such as "Praveen" as "pi-raveen", the word "ha","sha","ba" are generally ignored.