Their placement and duration are crucial to the understanding of a film. Shortening or lengthening a word by 2 or 3 characters can make or break the rhythm of a scene and, therefore, support or sabotage its meaning. Feel free to give to to any filmmaker contemplating the use of subs and inters. This goal is difficult to achieve because subtitles are texts that are read by the viewer while the viewer is to some degree disengaged from both the film's visuals and audio. Short films can more readily employ exactitude without risking audience fatigue.
Withdrawal leads to active resentment.
Choosing to subtitle impressionistically will often diminish the number of words viewers must read. The maker of a long film requiring subtitles must pull herself away from her loyalty to the content of the film in order to advocate for the audience by placing the film appropriately along that spectrum between exactitude and impression. The tendency, therefore, is to place them at the bottom border of TV-safe in order to keep the filmic space clear. The elements that determine if this placement is correct have to do with how many subtitles the audience will read, and over what length of time, and how fast the subtitles replace each other. This distance, over the course of a long film, risks fatiguing the audience to the extent that they stop reading the subtitles altogether, or, conversely, see little of the film except the texts.