Research on peer-production suggests that as participants choose what actions to perform, prototypical activity patterns emerge. Recent work characterized these patterns and demonstrated that informal emergent roles are highly stable. Nonetheless, we know little about the ways in which contributors take on and shed emergent roles. The objectives of this study are to: (a) delineate the temporal dynamics of participants' emergent role taking behaviors, and (b) identify the motivations driving role-transition behaviors. Our study links motivation to role-transition behaviors within Wikipedia. Our first sample covered eleven years and 222,119 contributors, and was used to identify four categories of temporal role-taking behaviors, that differ in their mobility between emergent roles and across Wikipedia articles. Our second examination linked the motivations of 175 new participants to their subsequent role-taking activity over 14 months. Together, the two analyses reveal that role-taking categories can be distinguished based on participants' motivational orientation (intrinsic/extrinsic and self/others-oriented).