Darwin's interest in emotional expression can be traced as far back as the Beagle voyage. He was fascinated by the different sounds and gestures among the peoples of Tierra del Fuego, and on his return from the voyage he started recording observations in a set of notebooks, later labelled 'Metaphysics on Morals & Speculations on Expression'. Details about the facial and bodily movements accompanying various emotions were entered, alongside remarks on the origins of language, aesthetic taste, and sympathy, comparing the behaviour of humans and animals. Darwin's work on expression was thus closely tied to his larger research programme on the modification of species and the animal ancestry of all our physical and mental attributes. Like the other notebooks on transmutation, these were kept private. A vast amount of research accumulated over time, and when the book finally appeared in 1872, it had been over thirty years in gestation.

Darwin sometimes referred to expression as 'an old hobby horse' and even made light of the work. 'I have got some funny notions on the subject', he wrote to Hooker in 1862. After Origin was published, he suffered an extended period of illness and despaired that he would never bring the project to a conclusion. At a particularly low point in 1864, he even offered all his material to Alfred Russel Wallace: 'I have collected a few notes on man but I do not suppose I shall ever use them ... There is much more that I shd like to write but I have not strength'. Darwin persevered, however. He planned at first to present all his conclusions on human evolution in a single chapter in Variation (treating man as a domesticated animal). But that book swelled to two volumes, and then so did Descent. By 1870, Darwin had amassed so much material that it required separate publication (Expression ran to 367 pages).

One of Darwin's main objects of study was infants. As soon as his first child (William) was born he began recording the boy's development in a separate notebook. But even before his marriage, he shared his interest in expression with Emma (then his fiancée), and asked her to observe the 'nods & winks' of her nephew, Ernest Wedgwood. The couple would continue recording observations of their children through the early 1850s. Other family relations and friends were asked to contribute. Perhaps unsurprisingly, many of the observations of babies and young children were made by mothers, and their communications were often addressed either to Emma or her eldest daughter. 'I am afraid you will think I have taken a long time in answering your questions,' wrote Margaret Vaughan Williams to Henrietta in 1869, 'but we do find it so hard to observe Baby’s expression accurately, & besides I seldom can attend to his eyebrows, when he is crying and I am wanting to comfort him'. Unlike the many men who contributed to Darwin's research, and whose authority as observers was often highlighted in his text, these women observers were rarely acknowledged by name.

While he was observing his children at home, Darwin was also studying expression in the family pets. His beloved dogs were favourite subjects and appeared frequently both in Expression and Descent. The family also kept cats, and Henrietta made a series of observations on their behaviour: one 'about the age of 10 months when put upon a soft cloak or gown used to take a piece in her mouth & then pound with her claws out & purr'. Observations extended to caged creatures. Darwin requested his niece Lucy Wedgwood to 'think of any fact about expression of any emotion in any of your birds.' J. J. Weir described canaries and goldfinches ruffling their feathers when frightened or angry: 'I have a Goldfinch male, often in my dining room, of the most irascible disposition'. The keeper of the Zoological Gardens at Regents Park was asked whether squirrel monkeys screamed like a baby, with wrinkled up skin around the eyes: 'Could you make it scream without hurting it much?' When in London, Darwin sometimes went to the zoo himself. One day he saw elephants shedding tears when they trumpeted, and he asked a correspondent in Ceylon, where elephants were captured and bound, to confirm: 'I am going to beg a favor of you, which will appear one of the oddest ever asked'.

Another main object of Darwin's investigation was indigenous peoples, especially 'savages' or 'less civilised natives' who had had little contact with Europeans. He began writing to naturalists, travellers and settlers in European colonies in the mid-1850s. His questions about expression were often conjoined with others on beauty and mate selection: 'would the handsomest woman in a tribe in the eyes of a New Zealander, be the handsomest woman in our eyes?' By 1860, his inquiries had become more systematic, and he sent a long list to Thomas Bridges, a missionary stationed in the Falkland's, including: 'Do the Fuegians or Patagonians, or both, nod their heads vertically to express assent, and shake their heads horizontally to express dissent?'. Darwin continued to refine his questions, and for ease of distribution he had the list printed around 1867, and began circulating it more widely through his correspondence network. Some of his contacts were able to distribute additional copies locally, so that Darwin's questionnaire reached some remote parts of the world. Responses were received from China, India, Malaysia, Australia, North and South America, and South Africa.

Despite Darwin's precisely worded questions, which could have been answered with a simple 'yes' or 'no', many of the replies contained lengthy descriptions of the natives peoples and their interactions with Europeans, some in highly charged settings such as criminal trials, or altercations with colonial officials in the streets. Observations of expression were often mingled with judgments about the emotional nature or moral character of the observed. 'Guilty sly and jealous expressions are distinctly seen in these two tribes notwithstanding that they are otherwise great adepts in concealing their feelings.' 'Defiance is indeed grotesquely expressed by the lower class of Bengalees. inter se. Again and again I have watched them in their brawls and squabbles, and really they strike me more as the snarling contentions of cowardly dogs'. Darwin received one response from a native in South Africa, the brother of a local chief who had been educated by missionaries and was employed by the colonial government as a constable. Christian Gaika's replies, written out in English, so impressed Darwin that he remarked to his colonial contact John Weale: 'That I shd receive answers written by the brother of a Kaffir chief is a truly wonderful fact in the progress of civilization'. Darwin's outlook was immediately condemned, however, by Weale: 'You speak sanguinely about the civilization of the natives, & the fact that Christian Gaika can write. This appears to me an error into which most people in England fall ...  the longer a Kafir has been on a Mission Station the worse servant he is.' Most of the overt racism of Darwin's correspondents was passed over when he came to summarize his findings for publication. Many of the cultural differences between peoples were also erased. Such exclusions allowed Darwin to conclude more readily that emotional response was universal across the globe, a key support to his more general argument about the common descent of the human races from a single ancestral species.

Another class of subjects whom Darwin thought were particularly important were the insane, for he believed that they shared with children the inability to control or conceal strong emotions. In 1869, he contacted the well-known alienist Henry Maudsley who forwarded Darwin's queries to his friend, James Crichton Browne, the superintendent of an asylum in West Riding. Browne was trying to make the institution a centre of research on madness and diseases of the brain, and he composed detailed descriptions of patients who suffered from emotional disorders such as extreme fear, rage, and melancholy. He also used photography to document and classify his patients, and he sent Darwin a large number of medical portraits, with his own hand-written diagnoses at the bottom. Darwin eventually used one of these, a patient named Ruth Lockwood whose hair bristled in horror similar to that of some animals when under threat. For Browne, Darwin's letters and the opportunity to assist him offered a 'genuine solace' in his 'house of bondage': 'The exigencies of the public service have already ruined my health, & curtailed my capacities. They now threaten to shorten my life'.

On learning of Browne's interest in photography, Darwin lent him another set of photographs that had been produced in a medical setting. This was the folio-sized album of Mécanisme de la physionomie humaine by the French physiologist Emile Duchenne. It contained over a hundred full-page images showing a wide range of expressions that had been produced using a galvanic technique. By applying electrical probes directly to the skin, Duchenne claimed he could reproduce expressions more authentically than the often exaggerated depictions of artists. When Darwin requested permission to use some of the photographs from his album, Duchenne refused to accept payment: 'So you will understand, dear Sir, that between men of science the question of money cannot exist.'  Darwin was extremely interested in Duchenne's work in part because it drew on the physiology of reflexes, rather than subjective emotional states, to explain expression. This was consistent with Darwin's theory that emotional response was fundamentally instinctive, a set of behaviours that had once given competitive advantage to animals, and that had persisted in humans, forming an automatic system of nervous discharge and muscular movement, such as the gaping mouth and eyes, the bristling hair and racing heart that accompanied fear and that (had once) prepared a creature to flee or fight.

Darwin drew heavily on the experimental approach to expression pioneered by Charles Bell in the early decades of the nineteenth century, even though he took issue with Bell's view that humans had a separately designed set of facial muscles to communicate higher moral and spiritual feelings. Darwin corresponded with a number of medical specialists to confirm some of Bell's findings on the physiology of weeping. He asked the ophthalmic surgeon William Bowman to observe if 'when an infant screams violently, it closes the orbicular muscles so as to compress the eyes & prevent them becoming gorged with blood'. The Dutch eye specialist Franz Donders performed detailed experiments on Darwin's behalf to determine the specific nerve fibres responsible for tear secretion: 'this is difficult and laborious, but not impossible... 'where there is a will, there is a way''. Some of Darwin's questions stretched the limits of physiological knowledge and challenged experts to investigate new phenomena. One particularly puzzling expression was blushing, which seemed to be uniquely human, and to involve the operation of mental attention upon the blood (hence its association with shame and embarrassment). Darwin found evidence of the phenomenon in dark-skinned races, even though it was scarcely visible, and among people who were born blind. But how could an act of self-attention (a pang of guilt), cause the face to redden? And how could such a stimulus-response mechanism have arisen?

One of the striking features of Expression was its combination of highly technical discussions (on the physiology of tears, for example), and deeply sentimental material (portraits of weeping babies). Darwin had started collecting photographs from the mid-1860s, when studio methods and technologies began to reduce exposure times sufficiently to allow emotional expressions to be captured with apparent authenticity. Darwin was particularly keen on portraits of children or infants smiling, frowning, or crying. He made contact with several studio photographers and succeeded in getting one of the best known, Oscar Rejlander, to work on commission. Rejlander took such an interest in Darwin's work that he sent detailed notes, posed for a number of pictures himself, and also took photographs of his wife, mimicking natural expressions. 'It is very difficult to get, at will—those expressions you wish ... In time—I might catch some —So I have tried in propria persona— —even cut my moustache shorter to try to please you in the last batch'.

At the time, the use of photographs was quite novel in a scientific publication, and it posed considerable difficulties. Darwin warned his publisher about the potential costs. He also tried to maximize the use of photos by mounting a number of images on individual plates. This had the effect of de-personalizing the images, making readers attend more closely to the details of outward expression (the shape of the mouth in crying, the furrowed brow in grief), rather than the inner feelings of the subject.

Though costly and time-consuming to produce, the photographs and other illustrations were extremely well-received by readers. Reviews also commented on the book's 'popular nature', the charming animal stories and entertaining anecdotes of children. Darwin's manner of involving his readers in the process of investigation may have encouraged them to share their own observations of expression directly with the author. For years after publication, letters arrived about dogs who grin, cats who beg for food, students wiggling their ears in excitement during lectures, children stomping the ground when angry, Maori men who rub noses and weep in the street. Darwin collected the material and made notes for revision, but a second edition was not published until after his death, edited by his son Francis. This may have been partly because the demand for reprinting slowed considerably after the first year, but Darwin was also keen to leave the subject of human evolution behind. Darwin's observations of children and his global survey of expression were among the very few areas of his research that focused directly on humans. After Expression, he returned at once to his favourite creatures on earth, plants, which would occupy him for the rest of his life.

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