David Razowky has a great little article discussing what to do when a scene has spun out of control. To summarize:

  • Make sure each line of dialog starts with "You ...", keeping the focus on each other
  • Be honest when confused or lost
  • Tell your partner how you feel about them
  • Slow the scene down - allow silences and work the environment
  • Don't let anything get by you - react to everything

I like every one of his suggestions, and they should be part of any good scene right from the start.