My co-author (Mark) and I have never met. We collaborate online to write, but we live in separate states. We've never seen each other face to face, and never spoken on the phone (or Skype for that matter). We communicate solely in text.
That said, I have more fun writing with a co-author than I ever had writing alone. Why? The answer is simple. Mundane things become hilarious.
Mark and I were writing a tear-jerker scene. Without giving too much away, a character was speaking with someone dear to him who had died. It was a little heart-wrenching. Especially when they started parting ways. Then, right at the most poignant part, what do I see Mark writing?
"And then, she burst into a fireball."
I nearly died laughing.
Now I can't stop writing, "And then X burst into a fireball," to Mark's dismay. I say it in conversations, which surprisingly enough works to elicit laughter.
Because I tease him so harshly, and because I have so much fun at his expense, and because he's so easygoing about it, every now and then, I just have to say thank you to Mark for making writing so much fun!