This paper presents an experience in laboratory dealing with the constitution of a corpus of multimodal spontaneous expressions of emotions. The originality of this corpus resides in its characteristics (interactions between a virtual actor and humans learning a theater text), in its content (multimodal spontaneous expressions of emotions) and in its two sources of characterization (by the participant and by one of his/her close relation). The corpus collection is part of a study on the fusion of multimodal information (verbal, facial, gestural, postural, and physiological) to improve the detection and characterization of expressions of emotions in human-machine interaction (HMI).