Collaboration is the making of a relationship not an object.
Although it sounds obvious to say it, collaboration is about difference, otherwise why bother. Acknowledging difference opens up a space to recognise what you don't know, what you do know and what you didn't know you knew; this, far more than the material outcome, is the substance of collaboration.
To be more precise about difference, the architect is legally constrained to know all there is to know of a situation, the architect operates through a convention of representation that brings objects into being, the architect is constrained by a duty to care about the client's desires. Artists meanwhile have faith that their most weird thoughts are relevant; they come from a tradition that has variously entitled them not to know, not to care, to care to the exclusion of everything else, to know and not to speak, or to speak with absolute authority. In contrast architects may go to prison if they don't do what they say they will.
Architects therefore achieve their ends by recognising the limits which constrain their practice and (hopefully) exceeding them. For an art and architecture collaboration to be successful - to make something that is more than either - the artist has also to recognise those limits and operate strategically in order to exceed them. Otherwise the architect will always assume the role of responsible parent to the wayward child.
Conversely, if collaboration is a relationship between differences, about what you give up and what you get, then the possibility of failure must remain real - for who really knows what they want and gets it? However the architect is not allowed to fail; the definition of failure for an architect is a legal not a philosophic one.
Katherine Clarke collaborated with muf in the Southwark Urban Design Initiative