A method for monitoring a mobile input device with a screen on which information can be displayed in a first pixel raster of image elements and which comprises a flat optical fingerprint reader and a second pixel raster of light-sensitive sensor elements. A fingerprint and fingerprint characteristics, comprising position of the finger on the screen are detected. The sensor elements detect the light intensity incident on them. The intensity levels are assembled into a static pattern of the fingerprint. The combination of the static pattern and the fingerprint characteristics are compared with a database. If the combination is in the database, a check is carried out whether an action is associated with this combination, which is then carried out, or whether no action is associated with this combination, whereupon a first standard action is carried out. If the combination is not stored, a second standard action is carried out.