I always enjoy new students in my classes, whether they're brand new to yoga, or new to Iyengar Yoga, or just new to my classes.
It's always interesting for me to watch people who are new to my classes, but who have some experience in other styles of yoga, or from other teachers within the Iyengar system. Sometimes they catch on quickly to how we do things in class. Other times I see some resistance to following my instructions and expectations. Or perhaps there's just not the awareness that they're doing something unusual for that class.
Each style of yoga has its own "culture" or way of doing things, and each studio and teacher may have it's own mini-culture as well. Just as you'd try to respect the culture and traditions when visiting a foreign country, it's a good idea to try to fit in with the expectations of whatever new yoga class you may attend. This shows respect and a willingness to learn. I like students who are respectful and willing to learn what I'm teaching!
You may think you know what you're doing, because that's how you've done it in other yoga classes....but that may be a misconception on your part for a particular class or method of yoga. Practice awareness in these new classes! The dedicated practice of yoga leads to awareness!
Be aware of how the other students are behaving in class. Follow along with what they're doing. Don't do something different unless you aren't able to do what's being taught. Don't do something different just because you feel like doing something different. Ask a question or two if you don't understand what's expected of you.
Part of the 'culture' of Iyengar yoga is to pay attention to what the teacher is asking you to do, and follow that as closely as you're able to. Don't "do your own thing". Watch when the teacher says watch, and do when the teacher says do.
Other styles of yoga have different expectations. Learn to pick up on what's expected in each particular class. Some classes may be very open to experimentation and doing what you feel like doing, while others are more directive. Follow instructions as closely as you're able to in those more directive classes. Err on the side of caution if you're not sure.
Why should you follow instructions exactly, or do exactly what the other students are doing? Why shouldn't you do what you're used to from your other classes? Maybe you think you can do a pose better in another way. Why can't you do it that way?
If you're following the instructions that I give, it shows me that you're paying attention, understanding what I'm saying, and open to learning what I have to teach during that particular class.
If I see that you're not following instructions it could show me that you don't understand what I'm getting at, or it could show me that you can't physically follow that instruction (so maybe I have to change an instruction), or it shows me that you think you know better how to do something -- that you're not willing to learn what I have to offer.
Part of the practice of yoga is to learn to be mentally more open. Don't rigidly adhere to what you've learned in the past. You won't learn anything new that way. It's fine to ask a few questions in my classes if you're really puzzled or worried about my instructions. But don't ignore them.
Practice being open to what a new teacher has to offer, and you'll learn so much more than ignoring what that teacher says. This is more respectful to the teacher and to the rest of the class. The way you normally do your poses certainly isn't "wrong" but it may not be appropriate in certain classes. Practice respect and awareness, and you'll get much more out of your yoga class experience.