A couple of times I've been in story meetings with the director, the producer and -- gasp -- the original writers. While this mark of respect for the original writer is admirable, it makes things more difficult. It's something akin to the new parents meeting the mother who's giving up her baby -- except that the mother has been FIRED for bad mothering. Or, if you will, a story meeting between ex-husband and new husband. You get the idea.
No one likes hearing what's wrong with their baby, er, script. No one likes it. Most people dwell on the problems, not the good points of a script. After all, the good points don't need fixing. But there are always good points. If you are meeting with a producer about someone's script, there are plenty of good things about the script -- no one hires a writer to fix a script that has nothing going for it at all.
The best rules to keep in mind are:
a. If you are the original writer, don't defend your own stuff. It is really important to say things like "I'm not defending what's up there, I'm sure we can find something better, but..." You are entitled to point out what the old version accomplishes in terms of setting up plot points or character, or entertaining the audience -- any replacement is going to need to do those things.
b. You can also say what you were trying to achieve. If they don't like the scene, it may be because it doesn't accomplish what you thought it did. But if you explain what you were going for, the other people in the room, if they're listening, can sometimes find a quick fix to your scene that allows people to keep it. Usually people would rather keep stuff than replace stuff. It's less work that way.
If you happen to be the new writer in this very awkward sort of meeting, don't be afraid to go overboard praising what's good about the old script. Don't worry, the producers know what's wrong, that's why you're there. Be sure to start with praise and end with praise. If the producers have gone to the trouble of having the original writer there, it's for a reason -- either personal connections or the writer's got some kind of guaranteed involvement. No one will mind if you "waste" a little time being extra nice.
And when you do criticize, be functional about it. "I have a problem with this because" or "this seems to cause a problem here" is better than "This is bad" or "I hate this." Try to draw out the original writer about what they wanted the scene to do. Take advantage of them being there -- draw them into the process as much as possible. Treat them as a resource.
If you're a producer thinking of setting up such a meeting, try to make sure that only one person is new to the current approach. In other words, don't have three different people weigh in at once. The writer needs to convince the director. Then the writer and the director sell it to the producer. Then the producer and director sell it to anyone else, with the writer giving out positive, thoughtful vibes. Otherwise people wind up talking at cross purposes. It seems like more meetings would take more time, but they can be shorter meetings.