Alternative history? Just guessing. My brother Chris used to read Harry Turtledove's alternative histories. That's about all I know about it. I do remember reading one where the south won the Civil War, and the author was a lot more hopeful than I was (having grown up in Virginia, where the war ended in 2008, when it voted for an African-American president!). But what a great subject, and let's face it, if you want to write FICTION about an important historical event, you need to go beyond the fact. So you won't get me carping that he was killed at night and so couldn't look out the window, etc. FICTION means, as Archibald MacLeish said about poetry, "not true."
The killer strode purposefully toward the President, knife raised high.
The President remained unaware, staring out the window into the oppressively hot DC night. His back was exposed, unprotected. My sister and I were immobile, too startled to react. I tried to shout a warning, at least give the President a chance, at least give the President a chance, but the words were stuck in my throat.
Why is he staring out at the night?
This is presumably NOT taking place at Ford's Theatre, where L was actually attacked, so see if you can sneak in some information, like "...staring out the White House window into...."
Can you sneak in a sense of where the narrator and sister are? "Immobile behind the couch" or "immobile in the doorway..." or?
at least give the President a chance, but the words were stuck in my throat.
At least give him a chance to what? Work on finding ways to slide in MORE. Here's the difference between narrative and reality-- in narrative, you can slip in more information with little words. :)
Also there is no reason to stick all that in one sentence. Conclusion, ending, results-- those are more emphatic in a single sentence at the end of the paragraph. Start thinking about sentence design and paragraph design as ways of letting the reader what goes with what. That's all about meaning too-- sentences and paragraphs. Don't assume that words and phrases are the real containers of meaning. Readers are sophisticated thinkers-- accept that, and assume they get meaning from what you put together in sentences, and what you put together in paragraphs. If you want the reader to know this is important ON ITS OWN, put it in a sentence of its own, and you'll be signalling: "Pay attention. This is important." So:
at least give the President a chance. But the words were stuck in my throat.
You don't need "were" there-- I'd decide on the basis of (you guessed it) rhythm. You can always vary rhythm by adding "nothing" words that don't really affect the meaning. This is the great skill that comes from writing bad but rhymed/metered poetry. No one who has ever written a bunch of sonnets will ever wonder what the rhythm of a sentence needs. :)
Helps to read Shakespeare and Frost aloud.
How did it come down to this, two kids trying to prevent this murder – a century and a half before their own time?
This is a nice first-personish way of quickly summarizing the situation.
I'd just suggest --
before OUR own time?
... makes it more personal. Those kids are "us", right?
Carin was between the assassin and his goal. He pushed her roughly out of the way with his left hand. She grunted as she spun around.
Opportunity to slide in setting info. Keep a watch for these, like:
Carin was between the assassin and his goal, there in the balcony seats... or there by the desk.. or?
In the beginning, the reader needs to have some sense of where we are, so slide in whatever info you can without being too obvious. This might be something you revise in. I tend to write in layers:
When you do your final draft, add in anything you think the reader needs (often only a word here and there). Then do another final draft and make sure everything is needed!
I saw her go down, saw the President still lost in thought, and before I could think about it, I was on the move. I jumped up on the President’s enormous bed, took two bouncing steps across it, and threw myself at the assassin. I grabbed him about the neck and upraised arm.
Okay, we're in his bedroom. The Lincoln bedroom! So... slide in info when you can with a word or two:
Where does she go down?
Is the president in his bed?
Block your action here. It sounds like she's on the other side of the bed from the assassin, but wasn't she/he just next to her sister?
I am really bad with action, but I try to compensate by story-boarding (with stick figures, natch) the movement. Where is "I"? Where is everyone else in the room? Make sure you know where everyone in the scene is positioned, because here, it sounds like "I" is bounding across the bed from the other side. It would probably take only a word or two to make clear where "I" is.
He glared ferociously at me and pushed me roughly back onto the bed.
Now how would you write this differently if "he" was on the bed too, and how would you write it if "he" was somewhere else in the room?
I am totally clueless about anything visual, so I truly can't imagine your action. That doesn't mean you should spell it out-- probably most readers are more visual than I am. But make sure YOU know where everyone is and what the action means. If "he" is on the floor beside the bed, he's going to reach UP to get to "I", right? Or "I" am going to fling myself from the bed and--- you're in first-person. What happens? Does "I" descend (as "I" would if "he" were beside on the floor) or not (if "he" were also on the bed)? And is Lincoln on the bed, or leaning on the window frame, or?
Block your action, and then decide what you need to tell the reader.
The President had heard the commotion behind him, and he turned back from the window. Even in the dim candlelight, his famous profile was unmistakable – the beard, the height, the gangly arms, everything but the stovepipe hat.
Past perfect-- "had heard"-- is problematic in several ways. If the Pres can turn in real time, no past perfect, better. Understand that the reader is going to cut you a little slack here, because we all know how hard it is to narrate simultaneous action. So if the president hears and turns in real time, that's good. If it's an instant before or after, the reader probably won't care.
The President heard the commotion behind him, and turned back from the window. Even in the dim candlelight, his famous profile was unmistakable – the beard, the height, the gangly arms, everything but the stovepipe hat.
Here's where you can sneak in a tiny bit more info about the narrator. How does he/she know what the Pres looks like? If you have:
his famous profile was unmistakable from the one in my history textbook – the beard, the height, the gangly arms, everything but the stovepipe hat.
the reader will instantly know that this is a schoolkid, right?
his famous profile was unmistakable from the portrait on Wikipedia – the beard, the height, the gangly arms, everything but the stovepipe hat.
...then we can assume that "I" is probably an adult.
The man aimed his knife at President Lincoln’s neck.
You call him "assassin" before, and I wonder if it might work to keep that? I'm assuming this is more complicated than just JW Booth getting an earlier chance, so you don't want to say "Booth".
Interesting opening. I am really interested in the "meta" aspects of alternate history, how much you rely on the reader's knowledge of what "really" happen (and notice I put "really" in quotes, like it's not really real ), and what you decide is "canon"-- what you aren't allowed to change, like the Pres's temperament or the year of assasssination?
(Later-- I actually hate one-sentence paragraphs, so I'd combine some of these. Assume no editor is going to allow a series of short paragraphs-- how would you re-paragraph this?)