First, I was very pleased to have a new player at this session. Trevor Kunkle, a friend from work, came by to watch a session and sample our playing style. If he likes it, he's welcome to return.
Rather than try to introduce a new player character at this point in the game, especially for someone who's not committed to the campaign, I gave Trevor the option to play Michael Chen. When it turned out that Michael didn't get to see a lot of action because of the direction the story took, I also gave Trevor a small Guest Star part to run.
The second difference was that Michele was not allowed to play her usual character, JD. I ran her as a Guest Star instead. Michele had been warned ahead of time to bring in a new character.