This article expands upon research into task planning effects by looking at how the addition of planning prior to group discussions may positively and negatively influence performance. 24 Japanese university students performed three weekly discussions in groups of four. Prior to each discussion, they undertook either no planning, strategic planning or an oral rehearsal with a partner outside of their group. Discussion were recorded and transcribed, and performance assessed using 14 measures related to participation, complexity, accuracy, fluency and discourse complexity. One-way repeated measures ANOVA results showed significant statistical effects on performance by both types of planning, especially with students who spoke less often than others within discussions. Post-hoc pairwise comparisons showed that rehearsal significantly improved participation (words spoken) and fluency (speech rates and pauses), while strategic planning reduced pauses and enabled students to give more reasons within speaking turns. However, strategic planning also resulted in students referring to the turns of others less often during discussions. These findings show that the addition of planning for group discussions can have a positive impact on participation and fluency, but that strategic planning may negatively affect how often students will directly address the turns of others.