Solution: The Secret Notebook at the Theatre

The clues

• 
  LenaThe painter who never finishes a canvas.

  She was in the Bar with another woman.

• 
  WalterThe estate’s groundskeeper.

  He was in the third column from the left. He was in the top row.

• 
  TheoThe paperboy.

  He was in the bottom row. He was in the fifth column from the left.

• 
  RafaelThe bicycle courier.

  He was in the third row.

• 
  Felix · VictimThe student who never sleeps.

  The victim. He was alone with the killer.

• 
  AlmaThe retired botanist.

  She was in the left column.

Step by step

The order our solver finds. Each step uses only the clues and what the steps before it proved, and its heading links to the technique.

1. Last cell left Every other cell is ruled out for Walter, by his testimony or by a row or column already taken. Placed: Walter: row 1, column 3 (Bar); Theo: row 6, column 5 (Stage).
2. Pinned suspects Rafael can only be in the third row, so nobody else can be there.
3. Pinned suspects Alma can only be in the left column, so nobody else can be there. Placed: Lena: row 2, column 2 (Bar).
4. Room counting The victim, Felix, cannot be in the highlighted cells: his room there could not hold exactly one other person. The cells meant: row 5, column 6.
5. Pinned suspects Felix can only be in the fourth column from the left, so nobody else can be there. Placed: Rafael: row 3, column 6 (Box Office).
6. Room counting Lena was in the Bar with exactly one other woman. Only Alma can still get there. Placed: Alma: row 4, column 1 (Bar); Felix: row 5, column 4 (Stage).