Each musician produces his own particular sound, not only through expressive patterns between successive notes (e.g. tempo and amplitude variations), but also at the individual note level, by the precise way the instrument is set into vibration. In the case of instruments for which the player has a direct mechanical action on the vibrating structure, this action represents an important part of the player's acoustical signature. Until recently, studies on musician's identity in individual tones mostly dealt with sustained instruments (violin, clarinet,...), where the player can modify the sound throughout its duration. However, according to musicians, this notion of acoustical signature also seems relevant for plucked string instruments. It means that, during the plucking action, the player gives initial shape and velocity to the string, which are characteristic of his musical skills or the technique he uses. The aim of the present study is to highlight characteristic parameters of playing techniques, dynamics, or musician skills, in the case of the concert harp. In order to analyse the finger-string interaction, a well-controlled experiment is performed with a panel of harp players playing in several musical contexts. The plucking action is filmed with a high-speed camera. Then, finger / string rotation and displacements are extracted using image processing techniques. These parameters will be used in order to define a set of musically-relevant descriptors of the musical gesture that can parametrize the initial conditions of the string vibrations depending on the skills of the player.