Readers like to see your characters in motion. Even a story focused on an interior dilemma can be made more gripping if the problem has some physical action to mirror it or contrast with it. As for stories focused on exterior problems, they’re like sharks — they can either keep moving or die.
Some writers have the storyteller’s version of kinesthetic awareness. They’re able to keep the complicated three-dimensional moving geometry of a multiple-character action scene running in their heads without having to work at it. When they need to describe what Character A is doing and exactly how that character occupies space and moves through the room in relationship to Characters B, C, and D, the only thing they need to do is check their mental diagram and it’s all there.
The rest of us have to work at it.
Diagrams on graph paper are one way to do it, and so is moving counters or figurines around on a tabletop.
Example: For a scene involving an infant, a nanny, a bodyguard, and a wicked kidnapper sneaking in through the balcony, break out a chess set. Pick out one of the queens to represent the nanny, one of the knights to represent the bodyguard, a rook for the wicked kidnapper, and a pawn for the baby in its crib. Designate one side of the chessboard as the balcony. Designate a square on the appropriate side of the chessboard in relation to the balcony as the door. Put the pawn in the right position for the baby’s crib. Then put your knight/bodyguard, your queen/nanny and your wicked kidnapping rook in their appropriate places — off the board next to the designated door square, on a square next to the baby pawn, off the board next to the balcony. Then start moving the pieces to block out the action, and watch how things happen.
(Amusingly enough, writers of romance and erotica can have similar problems, although with a different sort of action. They have to keep careful track of whose arms and legs are going where, and make certain that nobody ends up in a physically impossible position, or one that’s going to give them muscle cramps if they stay that way for very long. I’ve heard of poseable artist’s dummies being used to work things out, and have also heard of writers who’ve enlisted the aid of a sympathetic partner for the more athletic bits.)