Drawing on interactional pragmatics and membership categorisation analysis, with a focus on (un)accomplished
intersubjectivity, categories and social action, this paper explores some new aspects of multimodal joint fantasising in online
interaction. The data for this study comes from the public Facebook event page regarding the ‘ski field opening’ in Brisbane, a
sub-tropical city in Australia. The first part of the analysis examines how intersubjectivity is accomplished through joint
fantasising co-constructed among the posters, serving entertainment purposes. Invoking their membership in the category
‘fantasisers’, this is done in two ways: (1) flat co-construction gradient; and (2) upgraded
co-construction gradient. The second part focusses on the instances wherein intersubjectivity in relation to the
fantasy world is unaccomplished. It is indexed through (1) metapragmatic labels of humour types; (2) treating the event as real;
and (3) doubting the authenticity of the event and challenging the joint fantasising posts. As a result, additional categories
emerge, thereby constructing category relations, namely, oppositional categories such as ‘fantasisers’-‘the gullible’ and
‘fantasisers’-‘sceptics’. This in situ change, I argue, creates a shift in the pragmatic function of joint
fantasising, moving from a category-implicative action (serving entertainment) to a
category-relations-implicative action (serving jocular criticism). This paper adds to the research on joint
fantasising, categorial work and social action, and broadly contributes to our understanding of how members of the society orient
to contexts and categories in and through talk-in-interaction.