Production companies of today face extreme challenge to meet the rapid changes and increased flexibility that mass customization require. More and more customers are requiring the product to suite specific needs such as design, function and sustainability. These requirements results in increasing demands for the developers of the product but also for the personnel who will assemble the products in the final assembling. This paper suggests the need for further development, primarily addressing time parameters in dynamically changing assembly systems. We propose proactivity as a vital characteristic of semi-automated assembly systems, to increase fulfilment of customer demands and decrease non value-adding tasks. In proactive assembly systems, the potential of human operators and technical systems is utilised. Criteria for proactivity are reviewed from automation, information, and competence perspectives. Empirical data have been collected from five production companies in Sweden.