I always start with a general outline. It's almost never detail heavy, but it gets me going in the right direction.
Sometimes, that can lead to difficulty, however. Currently, in the Vampire Murders, I have a series of scenes, and I transition from one to the other. When one character went a little off script I ran into some trouble getting to the next one. To do it, I had to take a detour by completely shifting what I wanted to do temporarily to guide the characters into what needed to come next. That took me a couple of extra weeks (and then I had a hard drive crash and had to re-do it. And then they managed to save the computer after all, but I didn't want to give up the new stuff, so I've had to re-work it again to include everything. So now, I'm within one page of posting, and hopefully will manage sometime in the next few days. Whew!)
The point is, however, that if you run into a transition problem, stop, and see if you can detour your way into it. You may need to add a little to get there, or change what you intended to do slightly, but sometimes that's a plus, because you end up with more story, which is almost never a bad thing.
