Interesting topic.
I knew (know) a person who is no longer an elder for whatever reason, but he was known as a very good speaker. So he gets asked to go to congregations and give parts. He accepts these requests outside of his own state usually and gives parts. He tries to walk within the balance of his own convictions, but fails miserably at this. In speaking with him, I told him that even if he isn't technically SAYING anything that went against his conscience, when he came in as a "special speaker" he was just promoting that which he no longer believed in, the GB.
He likes to "teach" and I respect that. He didn't see the connection. I did. I haven't given a talk since my own conversation with this person.
Before when I was still serving, I just made sure I didn't teach anything agaisnt my conscience, and I would "switch" parts that I didn't agree with for a more benign part the following week or what have you. I just after talking to this friend of mine and seeing what he was atrempting, could no longer put myself in that position.
He said it was more for him than for them. The pinnacle of selfishness.