When my co-author and I decided which plot to go with and began writing, we found our biggest "fights" came from transition scenes. You need them. But they are pesky little creatures with a mind of their own. Sometimes you feel you've totally strayed from its purpose and the whole darn scene took on a major feel to it.
And there are so many questions. Should it be funny? Should it be suspenseful? Should it be short, long, and how much is too short and too long? Where is a natural ending? I mean a major scene you've thought out, has a main purpose. This is just supposed to lead from here to there, a setup for another major scene. In action thrillers, time for the reader to catch their breath. Since we are mixing some romance in this story, should that be the place for brief romantic interludes?
And just how do you deal with the very minor characters in some of these scenes? Just describe them? Do a little background? Don't even give them a name?
Add to all that the fact that my co-author just wings it. I'm usually the one agonizing over everything. For example, we both read a great series that had a major major character first described with blue eyes, then later brown...it made me nuts, she didn't even notice it. She keeps asking does it matter? And I keep answering apparently only to me.
Just the other night, we finished up a transition scene and took off a day or two for RL stuff. Came back together via IM and re read the scene. She loved it; I thought it was silly. Ugh. Those pesky transition scenes!