In order to do my PhD fieldwork among undocumented migrants in a detention centre, I had to become a volunteer for an NGO providing legal assistance. In this paper I examine the effect of this double commitment through the study of two figures: a `liar' and a `madman'. I question the grounds upon which field anthropological practice is based, namely, the ideas of long-term fieldwork and serendipity. I hypothesise that anthropological knowledge is constructed in the successive oscillations between various positions and points of view on the field and not in the quest for the right distance from the subject under scrutiny.