Christie's books are intricate clockwork, so people assume she built them at a desk with charts and string. She didn't. The hardest work happened in her head, often while she was doing something else entirely.

The Direct Answer
Mostly in her head, away from the desk, before a word was typed.
She worked out the clues and the culprit while doing chores or bathing, then committed the book to paper, and in later years to a Dictaphone.

The Thinking Came First

For Christie, the puzzle was solved long before the writing started. She filled notebooks with ideas, possible victims, alibis, and ways to mislead the reader, then turned the worked-out plot into prose at speed. A novel that took years of mental gestation could be drafted in a couple of months. The construction was the slow part, and most of it never touched the page until it was nearly finished. By the time she sat down, she usually knew exactly who had done it, how the misdirection would work, and which clue the reader would overlook in plain sight.

She was clear that ideas arrived off-duty rather than on-demand. 'Plots come to me at such odd moments,' she once said, 'when I am walking along the street, or examining a hat shop.' The desk was for transcription. The plotting was something she did while living her life, and she trusted her mind to keep turning the problem over in the background until a solution surfaced on its own.

The Dishes, the Bath, and the Apple Cores

The most quoted line in all of this is that the best time to plan a book is while you're doing the dishes. It's genuinely attributed to her, though its earliest print trace is a 1954 newspaper paraphrase rather than her own pen, so it's fair to call it commonly reported. The vivid version comes from a 1966 New York Times interview, where she described thinking out her plots in the old-fashioned rimmed bathtub and eating apples while she did it. The image stuck because it captures something true about how she worked, even if the exact phrasing has been polished by repetition over the decades.

I got my plots in the tub, just sitting there thinking, undisturbed, and lining the rim with apple cores.

Agatha Christie, as reported in The New York Times, 1966

Dull, repetitive tasks gave her mind room to wander toward the solution. Washing up, walking, soaking in the bath. The body busy, the imagination free. There's good reason this resonates with so many writers: the answer to a stuck plot rarely shows up while you're glaring at the page. Later, after she injured her wrist, she switched to dictating into a Dictaphone, then had a secretary type it up and corrected the pages by hand. She never warmed to hearing her own voice, and once admitted it made her self-conscious, but the method kept the books coming. The rest of her working life is laid out in the full Agatha Christie writing routine.

What You Can Steal