What we really need to do is co-host the class. Think of it as a balancing of expertise. Or two experts for the price of one. In small college settings, librarians can easily find themselves helping in classes or areas in which they aren't experts (or even have much content experience). Instead of seeing this as a weakness, use this to your advantage. Ask the professor to stay with you at the front of the classroom instead of escaping to the back. Have them interact with you. Ask them clarifying questions or let them introduce the assignment with you in class. Have them introduce you as another resource to take advantage of. And return the favor by praising their expertise in the field. (This works particularly well when discussing scholarly articles and just what kinds of people are authoring those studies!)
I'll be the first to admit this probably won't work with every professor, but I think for a few types of professors it will. Try it with that faculty member you know well and has seen you teach before. The one who knows how you roll. Or start with that faculty member who just can't help but interject in class. Take the interruption and make it part of the instruction. Or that professor who insists on talking with students during class. Make them have that conversation with you so you can guide it toward your instruction.
This idea comes from a colleague of mine who actually responded to my comment about him answering my question instead of the students with..."I was just helping out. We've got this thing going. It's like we're co-hosting this...like the Emmys." It made me and the students chuckle. Later, while the students were helping me brainstorm topics we had the following exchange.
"That's a great topic. I know someone, a friend, that writes about that topic."
"To be fair, you know someone who researches each of these topics. You know everyone."
"What can I say? I go to a lot of conferences and make friends."
Have fun with it. If the students are engaged they'll absorb the lesson better. Bring them into the research conversation by modeling a conversation. No one is expecting teaching (or comedy) gold. But may we all aspire to be the library version of that Neil Patrick Harris/Hugh Jackman duet on the Tonys a few years ago.