Students were observed directly, in a usability laboratory, and indirectly, by means of an extensive evaluation of responses, as they attempted interactive computer-marked assessment questions that required free-text responses of up to 20 words and as they amended their responses after receiving feedback. This provided more general insight into the way in which students actually engage with assessment and feedback, which is not necessarily the same as their self-reported behaviour. Response length and type varied with whether the question was in summative, purely formative, or diagnostic use, with the question itself, and most significantly with students' interpretation of what the question author was looking for. Feedback was most effective when it was understood by the student, tailored to the mistakes that they had made and when it prompted students rather than giving the answer. On some occasions, students appeared to respond to the computer as if it was a human marker, supporting the "computers as social actors" hypothesis, whilst on other occasions students seemed very aware that they were being marked by a machine. (Contains 8 tables and 11 figures.)