Prototyping is a well-studied activity for interaction designers, but its role in computer game design is relatively unexplored. The aim of this study is to shed light on prototyping in game design. Interviews were conducted with 27 game designers. The empirical data was structured using qualitative content analysis and analysed using the design version of The Activity Checklist. The analysis indicated that six categories of the checklist were significant for the data obtained. These categories are presented in relation to the data. The roles of externalization and internalization are specifically highlighted.