# Subject Motion — Video Generation

> How subjects (people, animals, objects) should move in video prompts. AI models handle motion very differently from static images.

## Motion types

### Subtle motion (RECOMMENDED for stability)
- Breathing
- Blinking
- Slight head turns
- Hair movement in wind
- Fabric rustle
- Eyes shifting

These work reliably in ALL AI video models.

### Moderate motion
- Walking
- Standing up / sitting down
- Reaching for object
- Turning whole body
- Talking (lip movement)
- Pouring liquid
- Opening door

Most models handle these but watch for artifacts.

### Complex motion
- Running
- Dancing
- Fighting
- Jumping
- Throwing
- Multi-person interaction
- Sports

Risky — models often produce distortions, weird limb behavior.

### Extreme motion
- High-speed action
- Acrobatics
- Vehicle stunts
- Crowds in motion

Avoid unless using Sora or Veo for short clips with careful prompting.

## Motion description language

### USE ACTION VERBS WITH MODIFIERS

GOOD:
```
She slowly turns her head from right to left, eyes scanning the room.
```

GOOD:
```
He gradually raises his hand to his face, fingers brushing his temple.
```

BAD:
```
She moves her head.    // Too vague
```

BAD:
```
He does a thing.       // No specificity
```

### USE "BEGINS TO" FOR NATURAL TIMING

GOOD:
```
She begins to walk slowly toward the door, taking three measured steps.
```

This gives the model timing cues — start state, action, end state.

### SPECIFY START AND END STATE

GOOD:
```
Starting position: standing still, hands at sides, looking forward.
Action: slowly turns head 45° to the left, eyes following.
End position: head turned, looking off-screen left, body still facing forward.
```

This gives clear motion vector.

### AVOID INSTANT TRANSITIONS

BAD:
```
He walks across the room.    // No timing — model guesses
```

GOOD:
```
He takes 3 slow steps across the room over the duration of the clip.
```

## Lip sync notes

If subject is speaking:
- Kling 2.x has best lip sync
- Veo 3.1 is acceptable
- Most others: avoid showing speaking from front (use OTS or back)

For dialogue:
```
Camera: medium shot from front
Subject speaks slowly, calmly: "I waited for you."
Lip movements should match this dialogue.
```

## Multi-subject motion

When two subjects move together:
- Specify each independently
- Specify spatial relationship
- Specify timing relationship

```
Subject A (left of frame) raises her hand in greeting.
Subject B (right of frame) responds by stepping forward and nodding.
Action timing: A's gesture begins first, B responds 1 second later.
```

## Physics constraints

AI models struggle with:
- Defying gravity
- Object permanence (object disappearing)
- Mass/weight realism (light vs heavy objects move similarly)
- Liquid dynamics (water, fire are tricky)
- Rapid acceleration

Use motion within natural human range.

## Default for AI video

When unsure: **minimal motion**. A subtle breath + slight head turn is more reliable than complex action.