In the present study, infants between 4 and 6 months of age were observed in daycare centers, either on their own or in the presence of another infant. Our goal was to explore the potential continuity between motor activity observed in early social situations and communication development. In order to investigate the influence of body posture on the opportunities of interaction with social environment, we also compared two conditions: infants were lying on a playmat or seated in bouncers. Our results suggest that infants modified their posture, movements and gaze behavior in dyadic situations compared to situations alone, reflecting a multimodal social tropism. Moreover, infants' positioning influenced the modality they engaged in the relationship with the other. This work highlights the need to analyse multimodal characteristics of interactions and to add posture variations as a key variable to better understand how infants develop communicative intention toward their peers.