Teacher Like An Angel

The Gemara states that one should seek Torah from a teacher only if he resembles an angel (Moed Katan 17a and Chagigah 15b). What the Gemara doesn't explain is: In what way is a
teacher expected to be like an angel?

I'm collecting answers to this question and hope to present some in a dvar torah. Does anyone have an interesting answer to share?

Thanks in advance.

Uri Cohen
uri-yo at bezeqint.net