An intentional motion blur portrait needs a decision about what should remain readable. You might want a recognizable face with a moving sleeve, a sharp pause within a turn, or a figure that becomes an abstract shape. Choose that intention before lengthening the exposure. It gives you something specific to inspect when the first trial comes back softer than expected.
Use the photography portfolio to discuss portrait framing before adding motion. For an evening concept, use the 3D artwork Blue Hour as a color reference while choosing a direction for the setting. Keep those choices in view as you test how movement changes the portrait.
Make a sharp comparison frame
Photograph the person with a shutter speed that stops the movement you expect, then inspect focus and exposure. This gives you a record of the composition before blur hides small problems. Check that the background supports the face and that the person has room to perform the planned action.
Explain the movement and demonstrate it without asking the person to imitate an uncomfortable position. Choose a clear start and end point. A slow sleeve movement, a head turn within a comfortable range or a step on a level path can all give you a repeatable trial. Let the person reset between attempts.
Keep the camera still for the first test
With the camera steady, subject movement is easier to isolate. Ask the person to keep their head reasonably still while moving a hand or part of their clothing. Lengthen the exposure and compare the moving area with the face. You may need several attempts because small differences in the action change the result.
If you want a turning face to remain partly recognizable, ask for a pause at one end of the movement. How much of that pause appears depends on the exposure and the pace of the action. Review the actual frame before asking for a longer or faster turn. A shorter, comfortable action is easier to repeat and compare.
A stable support can help, provided it is safe and permitted in the location. If you use a tripod, keep its legs clear of the person's path. Set up the motion so no one needs to step backward toward traffic, water or an unseen obstacle.
Test one exposure change at a time
For a slow hand movement, you could begin with a shutter time such as 1/15 second and compare it with 1/8 second. These are starting examples, not recommended settings for every portrait. The amount of blur depends on movement speed, movement across the frame, framing and camera movement. Even the same action may need a different trial when you move closer.
Keep the aperture consistent during the comparison if depth of field is already working. Adjust ISO as needed to keep exposure comparable, within the camera's available range. In bright light you may reach a point where the slow shutter makes the image too bright even at your chosen low ISO. Move to dimmer light, stop down if the changed depth of field is acceptable, or use an appropriate neutral density filter if you have one.
Write down the action and setting for the strongest trial. "One slow sleeve lift at 1/8 second" is more useful than remembering that a frame looked promising. Repeat it before changing the light or camera position.
Add camera movement as a separate experiment
Camera movement blurs stationary parts of the scene as well as the person. Try a small, deliberate movement in one direction during the exposure. Compare it with the still-camera version so you can see what changed. Bright background points will draw streaks that may cross the face, so check their path.
Panning follows a moving subject through the exposure. It can make the background streak while some subject detail remains clearer, though a walking person's head and limbs move differently. Choose a safe path with enough room, practice following the movement and begin photographing before the person reaches the chosen point.
Select for the intended gesture
Review the sequence at normal viewing size first. Does the blur describe the action, or does it obscure the part of the portrait you wanted to read? Then magnify the details that were supposed to remain sharp. A striking streak does not rescue a face that has lost the expression the image depends on.
Keep a few sharp frames from the same setup. They give you alternatives for a series and help judge whether the blurred photograph contributes something distinct. When several motion frames work, choose the one with the clearest gesture and the fewest distracting overlaps before applying a strong edit.