When painter Daphne Todd agreed to a commission for a portrait of Girton mistress Marilyn Strathern she did not realise that she would also be the subject of study. The painter and the sitter both give their perspectives.

By the time I met Marilyn Strathern last May, the preliminaries were over. Frances Gandy and Gillian Jondorf, official fellows at Girton College, Cambridge, had done their homework early in the year and approached me, through the Royal Society of Portrait Painters, with a view to commissioning a portrait of the mistress for the college. The budget was agreed and minimum dimensions established, although there could be some flexibility outwards.

Windows pierce three sides of Marilyn's spacious office, yet it is calm, quiet and dark inside. The woman I had come to meet appeared small and pale within it. On entering, I instinctively turned towards the source of a warm glow - the reflected light from the Victorian brick walls outside, filling one window entirely. I held on to that.

I suppose we discussed how long she would pose (a minimum of 15 hours) and where, in the usual way. Marilyn expressed a small degree of anxiety over the outcome, as women (though not men) always do. I wondered if it would help to be part of a more complicated picture, one with a view through a window, for example.

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"I do not want a conventional portrait with a view," she said.

"Not a view-type view," I said, thinking of the glow.

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"Something off-centre," Marilyn said and, with this key phrase, the light shone in.

"I may do it on two panels," I said.

I had been wanting this freedom since painting a difficult non-commissioned portrait for a special exhibition, but not even my more colourful clients had yet been willing to risk it.

After lunch, Frances Gandy patiently showed me the college collections and the various rooms where sittings might take place.

"Were you able to observe the mistress?" she asked. "I had the feeling she was studying me, minutely," I replied.

"Ah, you noticed. Not everyone likes it."

The tour ended with tea in Marilyn's own cottage. It has a curiously placed window that induces the feeling that one is Alice, grown too large to properly look out.

"Dinky," I said, much to Marilyn's annoyance, and decided upon the college setting.

As I left for home, dates fixed for August, Marilyn handed me 14 pages entitled "Prefigured features: A view from the New Guinea Highlands" (M. Strathern) "Stimulated by a conference on Portraiture and the Problematics of Representation".

Oh Lord, I thought, these Oxbridge types always know more about my subject than I do. My admittedly limited understanding of one of the arguments contained therein is that in one Papua New Guinean culture, severed heads represent the individuality of those who have severed them rather than those to whom they once belonged. Marc Quinn, "Young British Artist", might be interested, but I work on a different (and much lower) plane.

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Daphne Todd's portrait of Marilyn Strathern won the Royal Society of Portrait Painters' Ondaatje Prize for Portraiture 2001 Anyway, giving Marilyn two heads seemed appropriate. I cannot remember precisely what led to it - although it would not surprise me to discover that the seed was sown deliberately by the sitter. The painting developed in a straightforward way. In representational painting, the illusion of light falling on forms depends upon the relative value of all the tones used. I could not simply "cut out" each head and expect the illusion of depth to be maintained. So each head relates to its own bit of background - its own space. These spaces collide. They could have overlapped, but the painting itself makes demands in the same simple way that flower-arranging does: balance and counter-balance. Straight diagonal lines began to emerge.

"I do not know where this is going," I said.

"I like straight lines," Marilyn said.

I think Marilyn posed for longer than she expected - partly because she was prepared to, and I took advantage, and partly because there was, of course, twice as much to do. (Advantage Girton: two figures for the price of one.) We worked morning and afternoon from Monday to Friday, and I alone continued into Saturday, by then concentrating on the sunny, brick view that I had left mostly to the end, guessing it would outlast the sitter. This part of the painting is on a separate panel because for me to see the window with all that orange light required a 180 degree turn, so it was virtually impossible to relate view to figure in other than a cursory way. Months later, the artist John Ward CBE was to rap me over the knuckles for "so very nearly compromising the space around the head with that jump in the picture plane".

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I am writing as though our encounters were largely silent. I think the posing/painting was. It takes two to make a portrait and Marilyn put every effort into giving me what I wanted. Views were exchanged during breaks and, memorably, over an Indian meal one evening. I was wide-eyed over Marilyn's experiences in Papua New Guinea and indeed as a lone mother of three. But I doubt my knowledge of her courage and stamina affected the development of the painting. My response is visual; words get in the way. Inadequate though it almost certainly is in the face of Marilyn Strathern's achievements, I cannot add to the painting by way of explanation. It has to do its own work. But I could always have another go.

Daphne Todd's portrait of Marilyn Strathern won the Royal Society of Portrait Painters' Ondaatje Prize for Portraiture for 2001.

When a sitter finds herself in the way of the big picture

There is not really anything to say. An earlier attempt to put the sitter's experience into words failed miserably, but, given Daphne Todd's text, I feel better able to respond with like for like.

It was amusing that my initial offering of words also failed. Daphne was pretty scathing about the article - it was on assumptions that lay behind defining a portrait as "any body representation that stands for an individual" - and with a bit of a shock I realised that she was using the word "conceptual" as a criticism. I think that in offering the article to her I was trying to do something akin to networking, finding a way through the skein of chance encounters to make a point of contact. And the chance encounter here was that I had actually, if very briefly, taken up a commentary on institutional portraits by a colleague (Charlotte Townsend-Gault, department of fine arts, British Columbia) as grist to my comparative mill. Only I had definitely chosen the wrong medium as far as Daphne was concerned: it was the anthropological analysis she saw, not a commonality of subject matter (institutional portraits), and that analysis seemed to get in the way. So also her seeing me "studying" her: knowing I was a social anthropologist somehow got in the way.

Getting in the way of the flow of life is what anthropologists do when they elicit the shock of recognition, whether through making the familiar strange or the strange familiar. They may try to convey this to others; they can also find it happening to themselves. I was unprepared for just how foreign the experience of being painted would turn out to be. I do not mean being rendered familiar/unfamiliar - I mean the nature of that five days of activity with Daphne.

I was indeed watching her, but not "her". Contrary to popular belief, anthropologists do not study people: they study what people do and say, that is, culture and society. And what totally absorbed me were the movements of the painter's hands and eyes. In fact, I found them mesmerising. A feeble attempt on my part the first day to remain in a familiar world also failed. I had somehow thought that the process would be a fairly mechanical one and I could get on and catch up with some reading. Instead, I found myself unable to concentrate. My attention became locked into what was happening there and then; in short, I had to abandon myself to the unfamiliarity of the occasion. Which, of course, is what happens to anthropologists in the field who take their cues precisely from what happens to them. As students find, the process of actively abandoning previous ideas is of supreme importance. Perhaps this was an active abandonment.

Certainly it was something of a take-over. Daphne had begun her preliminary sketches with the raised central head; it was because I had bent it to read at one point that she saw the second portrait. So instead of me cleverly managing alternate real-life positions - posing (head up) and reading (head down) - the two heads were drawn into the painting itself, and it was all pose.

Daphne has a little underrepresented the role of words in the posing. They got in the way in the most animating sense. Daphne was extraordinarily open and generous. She shared with me the emerging picture, which I was able to observe as it happened, and described what she was doing with colour. With the tiniest of brushes, she gradually covered the birchwood on which she painted. Each stroke was intended to be true to what she saw. Coordination and composition seemed to summon themselves through the simplest application of colour, except that it was not simple at all. As she explained, each individual application must work alongside all the other applications, in their company so to speak, and it is from this that the effectiveness of colouring comes. There has to be, she implied, a constant translation between what is observed and the place that each minute brushstroke is going to have within the frame. I could not see the difference from words - each requires attention, and each changes in its context with others, takes on character from the words around - but that conceptual leap of mine did not please either.

Getting in the way could almost describe embodiment. In a sense, it was I who was in the way. The fellows of Girton College seized the opportunity of my being on leave last year to pin me down, which is why the portrait was done so soon after I became mistress of Girton. Between the institution and the portrait painter, the sitter for the institutional portrait is a momentary filter, an embodiment of office. In this case, the image belongs to the fellows not just as property in which they have reproduction rights, but as an image of an office. Any particular portrait sits in the general company of portraits on an institution's walls. So consider the embodiment we take for granted in a body representation. Embodiment, taking an individual form, is crucial to creating an image in the first place, and this is perhaps the sense in which the sitter gets in the way. The office is always inhabited, and the image can only be of the inhabitant.

Incidentally, there is nothing impersonal about being this kind of body. On the contrary, being very much a person is part of what the office-holder must be, and it is absorbing. So it is most appropriate to record just how I was drawn into the whole thing. Bit of genius there on the painter's part.

Dame Marilyn Strathern is mistress of Girton College, Cambridge. Daphne Todd's portrait of her is on display at the National Portrait Gallery until September 9.

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