Recently a friend and I were discussing a script and this exercise again cropped up. The instruction we were given before shooting for the exercise called 'interview' was to choose any person and extract the maximum information from them. What the ten of us in direction can be read here.
I have great respect for this exercise and believe this is something you should do before beginning on any screenplay (ideally any sort of) writing. Like our professor pointed out rightly, in most scripts we see a trend that ALL characters speak like the writers themselves. Not in terms of dialect etc, but characters are all drawn from the writer. This is not that creative. When you simply film people around you and later listen to them talk, you will see how different they are from you. This difference should be given ample representation in your writing.
Even if its not for this purpose its great to film people talking. Once in a while you can watch them and observe all their mannerisms and other ways of conducting themselves. People are always interesting. Well, most of them are.
Don't wait. Just film people talking. Go interview.
This here is Babu, a cook in a mess nearby, run by some malayalee nurses.