I met some yoga instructors who weren't very responsible and knowledgeable. What we may think of the good roles and responsibilities of a yoga instructor? Well, as a yoga instructor one should not only be concerned with the supervision of exercises and fitness routines, but also should play a multifaceted role of an advisor, supervisor and evaluator of several activities taking place during her/his work.
We may think of these:
1. Planning and leading exercise, asana (
asanas -- the Indian word for the yoga poses) or activity sessions for the clients.
2. Evaluating client questionnaires and physiological data to determine appropriate exercise/activity regimens.
Don't be like those ones who even don't ask who has some waist problems! 
3. Planning individual exercise/activity programs for clients in such areas as cardiovascular conditioning, flexibility training, yogic postures and ensuring safe and complete client training.
It's not only learning yogic postures!!!4. Setting and monitoring training equipment. Ensuring that equipment is returned in serviceable condition.
5. Advising the clients on program and class policies and procedures, as appropriate.
Not only thinking and talking about the money!6. Assisting in performing follow-up and communication with clients.
Not just thinking when you'll say "good-bye"!7. Tracking clients' progress and maintaining complete and accurate client files. (
This will help about the monitoring client's performance and helping her/him to target a yoga goal.)
8. Ensuring clients' adherence to safety and injury prevention policies and procedures! Enforcing safety procedures.
9. Demonstrating various exercises and helping clients to improve the exercise technique.
10. Imparting knowledge and implementation of the principles of yoga.
11. Passing on technical yoga information to non-technical people.
12. Taking good care for the yoga room/hall/place. Ensuring the security.