Previous empirical studies in sketches and mental imagery showed that there is no significant difference in overall quality and possibility to use mental imagery as design tool. This preliminary study explores distinctions between two kinds of sessions in terms of how ideas are generated. Four design sessions of two novice designers are used to unveil differences. Based on preliminary results, physical properties of sketches underlie differences, also the availability of visual cues apart from the drawings itself. During interpretation stage, sketches provides an additional dialogue which is not available in mental imagery session. The use of mental imagery as design tool in novice designers vary and may not as effective as in experts. Pauses and gesture in both sessions are found to be fundamental designing aspects, including in environment when sketches are allowed. When crucial differences are no longer assumed, interplaying roles between the two can then be explored further.