I'm thinking about starting a novel by having the first sentence be internal dialogue.
What are the potential problems this could raise? What are the benefits?
By internal dialogue I mean something like:
It's so high up here. Thought Sam while...(sentence continues)
There's no reason why it couldn't work, as long as you quickly make clear that it's internal dialogue. If it's a first-person narrative, the entire story is "internal dialogue," in a sense.
The main benefit is to give the reader immediate access to the character's inner life, which may help us identify with him/her/it/them.
The only real con I could see is if it's written in such a way that the reader is confused whether the text is narration or internal monologue, or if you can't tell who's speaking/thinking for a while.
Your title, first sentence, paragraph, page, and chapter are your hooks to catch the reader. Make sure they are baited well. So, if you start with internal monologue, it had better be interesting, not just bland, random thoughts about how it's high up on the wherever. I realize that was just an example, but compare:
Bad: "It's so high up here," thought Sam as he climbed a tree.
OK: "It's so high up here," thought Sam from the claws of the giant eagle.
Better: "I can see the whole Shire," marveled Sam, then screamed as he began to fall.
I've read several stories where this sort of technique is employed. The most memorable had something to do with the character pondering their own death, and the rest of the story consisted of the events leading up to it. It started with something like
I hadn't expected to die so young.
I admit this might have worked purely because of the shock value of the first sentence, but I don't think that it was just the shock value. It was neat to be inside a character's head starting at the very beginning. The reader gets to know the character very quickly that way.
The main difference between this example and what you proposed is that your example is in third person and this example is in first person. I think you might have to play with the structure more to have it be a strong hook in third person, and you'd have a lot of quick introduction to make, but it could definitely work.
