I am currently writing the third book of the fantasy trilogy (again, WHY do they insist on being trilogies?) and I thought I'd pass on some insights into what works for me when I am writing a big fat book.
Unless you write like James Patterson with 45-page outlines and then send your interns off to finish it because you've already written the whole thing in your head, OR you write like Dean Koontz and perfect five pages before going onto the next five pages and there is no outline...
You may get stuck in your story at times.
It happens to the best of us. Some call it writer's block. We have an idea of where the story should go, and some key scenes that will be boffo for the reader, but we get lost in the woods of words and don't know how to get there from here. I actually heard a would-be writer once say, "I know all the key scenes, and the rest I'll just put in filler."
Don't ever do that. I will hunt you down and slap you if you do.
Here are some things I've done to avoid getting stuck:
1. Re-read the story. If it's a long manuscript, I usually just re-read from the last action sequence. It's like jogging my memory, the way I retrace my steps when I've forgotten why I'm in the kitchen. I read it and think, "Oh, yeah, THAT'S where I was heading with this."
2. Have my character do something mundane. Eat a meal. Wash clothes. Take a walk. This makes my character go forward in their space, making it easier to get them where I want them to go. Ninety percent of the time I delete all the boring stuff, but every once in awhile I find that little nugget of a revelation/clue/foreshadowing that stays.
3. There is the Soupy Sales approach (aka "and then the dragons came"). Soupy Sales was a guy with a kids' show in the 50s and 60s. At some point in the show, there would be a knock on the door and it would always be a surprise guest to mess with him. If you don't know what else to do, throw a surprise at your character. A visit from the suspect's mom. A door that leads to a room with a clue. An unexpected gryphon. Hit them in the head with a golf club (one of my favorite moves). Again, it might not stay in the manuscript, but it gets your character moving forward.
And don't forget to treat this like that first blank page of your manuscript:
1. This is a rough draft.
2. No one will see it except me.
3. I can start anywhere, even with "Okay, this is the story I want to tell," and ramble on until the tale starts coming out.
4. Once I've typed the end, I'll have a better idea of where it should begin.
5. I CAN'T EDIT A BLANK PAGE.
There is one caveat to all of this: as you work your way through the stickiness, you may not find a way back to that boffo action scene you had planned.
It's okay--there will be another boffo scene to replace it, one that's bigger and boffo-ier.
I hope I've been helpful. Just remember, use what makes sense and discard the rest. As always, your mileage may vary.