1) Yes ,you can choose subjects of your interest ,provided you are not violating the FPSC rules of Subjects selection.(Do waste a minute to read them in syllabus)
2) You have to attempt papers for total of 1200 marks (6 papers compulsory and 6 papers optionals )
3)Preparation time varies from person to person .(Many factors are at work here)..some people have done it in 2 months and some in 1 year ,,,But average time is 6 months to 1 year
4) Writing expression (simple and appropriate expression/vocabulary not flowery and difficult vocabulary which the writer himself cannot pronounce correctly ) is the basic and defining factor in CSS.

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Power is dangerous ,for it attracts the worst and corrupts the best !