• Expressive Strokes? No. - Hold down the Shift key and the lines can be painted at 0 degrees (horizontal), 90 degrees (vertical) and 45 degrees.

• Expressive Strokes? No - Click the Straight Line Strokes icon on the Property Bar (third icon from the left) or click the V key and you can paint a straight line at any angle you want:

1. Click at the beginning of the stroke.
2. Click and hold near where you want the stroke to end.
3. Then drag to the exact point where you want the stroke to end and let go.
4. Click again to continue the straight line stroke at another angle.
5. Click the V key again to begin a new straight line stroke.
6. Click the B key to return to freehand strokes.

• Expressive Strokes? Yes - Use the Align to Path feature:

1. Create a straight line Shape using the Pen tool.
2. With the Brush Tool active, click the Align to Path icon on the Property Bar (fourth icon from the left).
3. Paint follow the Shape's path and, in the Layers palette, the brush stroke appears on a Layer above the Shape.

• Expressive Strokes? Difficult at Best - Lay a ruler or other straight edge on your tablet and use that as a guide for your stylus. I find this cumbersome and it would definitely not be my method of choice.
  
  
• Expressive Strokes? Yes - Practice, practice, practice to paint straight lines without the use of other aids beyond your own eye/hand coordination. It can be done.