Hi

If you are applying in IISER by appearing in aptitude test conducted only for SCB channel candidates, there will be no interview. You just have to qualify in the test. 

As far as I know, you will have to appear in all the four subjects in the aptitude test - Biology, Physics, Chemistry and Mathematics and yes, the exam is not that tough to crack. Just be thorough with your class XII syllabus and be clear about the concepts.

To know more about IISER, check the links below:

IISER Syllabus and Exam Pattern

IISER Admission 2019

All the best!