One-on-one D&D - A DM's perspective

Since we don't currently have any campaigns together, ShoshieB1 asked me to run some D&D for her one-on-one. Running an RPG one-on-one present some unique opportunities as well as some unique challenges. In this story, I'm going to discuss some of them and what I think it'll be like.

Opportunities

• Single-player focus - Since there's only one player, I don't have to worry about giving multiple people a chance at the spotlight during a session.
• Scheduling - I can think on my feet well enough and have enough of my world fleshed out that we can play whenever both of us have some free time.
• Intra-party conflict - In pretty much every campaign I've played where there was PC vs. PC conflict, someone's feelings almost always got hurt. With only one PC, it becomes possible to have conflict with the NPC "party" without the player conflict.
• Fleshing out the setting - Since one PC can go anywhere and do anything, it gives me a chance to flesh out places, NPCs, politics, religions, etc. that I haven't done yet.

Challenges

• Off nights - If one of us is having an off night with our role-playing, there is no one else at the table to help keep things moving. As a DM with muliple players, I can get the party discussing things amongst themselves in-character. As a player with multiple players, it's possible to sit on the sidelines a bit.
• No back-up - I have to be careful with the life-threatening challenges I send ShoshieB1's way since there won't always be other party members around to cast a cure or stop the bleeding.

Has anyone else run a one-on-one RPG campaign? Any tips, ideas, or thoughts on your experience?