The problem here is that knives are a lot like guns when it comes to lethality, they possess the capacity to deliver crippling injuries and almost instantaneous death even when someone doesn’t know what they’re doing.
Outside of the intimidation factor, for a character who knows what they’re doing dealing with someone who is larger and physically stronger than they are isn’t really a big deal. Especially if they don’t rely on brute force to get their job done. Facing someone who knows how to lock you up, break you up, and take you out is much more frightening.
If these two characters don’t know what they’re doing then that physical strength is a game changer. However, all the muscle in the world isn’t going to protect this character from a stab wound.
The easy thing to do in the sequence is have your smaller character lose the knife. At some point early in the scene, they lose their grip on it and it falls to the floor, gets kicked away, or goes flying off. Either way, they lose it quickly as many people do when they’re nervous, scared, and the adrenaline pumping through their veins makes them jittery.
If they lose the knife then they may see themselves at a disadvantage, the struggle to regain the knife will put them into a position where they aren’t focusing on their opponent and may almost get killed only to recover the knife in time to finish their opponent off.
It’s not so much a fight scene as a mad dash scramble, but that’s all fights really are anyway.
Whatever way you go, here are some things to remember:
Introducing a weapon into a fight scene is more than just an ancillary choice, by arming your character you place the focus on the weapon itself and the weapon will drive the fight. The knife provides a greater advantage and is visibly the most dangerous quality the character possesses. Any intelligent person will seek a way to negate that advantage if they can and a stupid person may want to ignore it (like not going after the knife once it’s no longer in the character’s hand) but it only becomes non-threatening after the asset is negated.
Regardless of the characters wants, desires, and intentions, the scene revolves around the knife.
Physical size and strength are only imposing if you write it that way
In all honesty, physical size in a match up like Jet Li versus Dolph Ludgren in the Expendables is a visual gag. Too many people assume a big, brawny, burly person who lifts a lot of weights is automatically a better fighter (unless they’re Asian, thanks Hollywood stereotypes!) by virtue of size and muscle. As I said above, it’s not actually true. A very large person is not automatically imposing or threatening, especially not if the character is used to combat. I personally find sparring someone or working with someone who is taller and more imposing than me to be very comfortable because most of my instructors and students I trained with were larger. I know what to expect, more or less.
What I’m saying is that while movies use it as a visual fast hand, it’s not an automatic advantage. It’s also not going to help you in a written context unless you make a point out of it. Personally, I blame D&D for defining combat skill via a strength score but given we get questions about size and strength a lot… stopping myself from pounding my head into a desk is difficult.
Use the environment as an active participant in the scene
Forget about size for a minute and think on what both characters bring to the table. Think about where they are fighting and how their environment can be used to add tension or force the characters to make difficult choices. If the knife gets kicked under a nearby dresser does Character A dive and go scrounging for it or do they try to fight Character B? Either choice is legitimate from a narrative perspective, but what they choose to do with their options will open up a pathway to getting your imagination pumping.
It’s easy to get too caught up in trying to figure out how a punch or kick works and describing it while forgetting about the surrounding environment. Different environments yield different advantages and may be a more comfortable place to start.
Fights don’t happen in a vacuum, by starting with somewhere concrete like a location you can have the character’s assistance in figuring out what they would do at that specific location. By over focusing on the major players, it’s easy to forget the other variables involved. A shootout at a shopping mall has different priorities than a shootout on an abandoned bridge, a fight in a chemistry lab will look different from a library, fighting in a park isn’t the same as fighting in a warehouse.
There is no pause button
The countdown clock on the events in the surrounding narrative don’t just hit pause just because the two characters have hit their climactic fight scene. The priorities each character has will govern how they fight as much as what they know how to do.
Define the goals of each one. Are they on the clock? How fast does this need to be over? What’s at stake? Do they want to kill their opponent? Are they playing for time and stalling until help arrives? Do they think they’ll win? Do they even plan on living through this? (The answer can be no.)
A character who enters a fight desperate and disappointed will be different from one who has just come off a string of victories. Don’t think about the fight in terms of what someone else who knows how to fight would do, think about it in terms of what your characters would do. What they do ultimately comes down to who they are and what they want. Fights are personal, even when they’re not.
I know it can feel daunting when you’re still trying to figure this fighting business out, but the best way to get set for your scene is to eliminate what’s not important. What other people do out there in the real world is not important, what your characters choose to do in this singular moment is.