I recently read part of the first draft of a novel where a 30th-birthday dinner was to end in conflict. This one didn't. The scene was well-written and the characters distinct, but the scene was flat as a newly tarred driveway. Why? No tension.
A scene can lack tension for various reasons. In this case, it was because none of the characters was allowed to react to the discomfiture of a main character when someone inadvertently reminds him he was away from his wife when she was dying of cancer. Actually, it may be more accurate to say everyone's reactions to the moment were subdued. It made the scene and the characters appear dull, one-dimensional.
The other reason there was no tension was that the protagonist's own emotions were muted. What did he really feel? How would he show that? How would others respond to his pain, especially his daughter-in-law, whose birthday they were celebrating? Would she feel empathy? Would others sense something is wrong but not be sure how to respond? What about the man's date-does she long to reach out to him but can't because he won't accept her love? What about his son? Does he feel guilt because he was with this mother when his father wasn't? Since he's a doctor, was he complicit in his mother's passing?
In this case, the universal lack of response drained the life out of the scene. Yet, characters-I like to call them people-long to get out of their shells if we'll let them. We don't need permission to write the truth; it will set us writers and our characters free.
Tip: The fix in a case like this is to revise by re-visualizing, re-visioning the scene, if we can use those words as Natalie Goldberg did in Writing Down the Bones. Start with a clean sheet of paper or a new document, and close your eyes. Allow the scene to materialize, and watch each person respond. This will deepen the scene and broaden it. For more on this technique, see the post "Stephen King to the Rescue: Using Imagery to Bring a Story to Life." And see the August issue of The Writer, the magazine archive piece by Stephen King called, "Use Imagery to Bring Your Story to Life."