Co-sponsoring of policy proposals has received limited scholarly attention in the context of parliamentary systems, where the legislative process is heavily dominated by the government and the vast majority of enacted policies are initially proposed by the cabinet. In this study, we analyze why cross-partisan co-sponsoring of private member bills occurs in parliamentary systems. We argue that such interparty policy work is significantly more prevalent under minority governments than majority governments, as parliamentary cooperation becomes more necessary and the likelihood of success increases. Furthermore, we contend that cross-partisan co-sponsoring is influenced by both MP's ideological proximity on the issue and the salience of the policy. Utilizing a comprehensive dataset on private member bills in the Norwegian parliament combined with individual MP characteristics, we test these arguments empirically. First, we show, through network analyses, that MPs are far more likely to co-sponsor bills across parties during periods of minority governments than majority governments. Second, we scale PMBs to pre-defined text embedding centroids based on party programs to show that cross-party co-sponsorship is favored over single party sponsorship on salient issues where MPs are ideologically proximate. Finally, we utilize within-period cabinet restructuring to show how MPs utilize PMBs to build coalitions in parliament before entering cabinet. Our findings contribute to a deeper understanding of legislative behavior and coaltion building dynamics in parliamentary systems.