There won't be any interview rather a Selection Board would be held for that post. You shouldn't worry if you're at home in your subject of specialization.
Tips:
1. Choose such topic over which you've a strong command.
2. Use simple diction and idiomatic style that is easily comprehensible.
3. While demonstrating, feel like you're teaching to a class of students.
4. Don't try to over-act.
5. Be confident, for confidence is the currency you can exchange success with.
Regards

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"Success is not final, failure is not fatal; it is the courage to continue that counts" (Winston Churchill)