This article discusses the internal architecture of the Mid-ifilePerformer application. This software allows a user to follow a score described in the MIDI format at its own pace and with its own accentuation. MidifilePerformer allows for a wide variety of style and interpretation to be applied to the vast number of MIDI files found on the Internet. We present here the algorithms enabling the association between the commands made by the performer, via a MIDI or alphanumeric keyboard, and the notes appearing in the score. We will show that these algorithms define a notion of expressiveness which extends the possibilities of interpretation while maintaining the simplicity of the gesture.