CLOSE-UP
The close-up is an important element of movies. It's a more difficult shot to do well than the long shot or the medium shot.
It's more difficult on a technical level. Just keeping an actor in frame and in focus are difficult jobs when using telephoto lenses; but, that's not the main difficulty. You should know when to use a close up.
If you use a close up too late in a scene, and you're holding on actor's profiles in a two-shot, or a distant long shot, the viewers are craning their necks and squinting their eyes to see who's talking, what's their expression or who are they anyway!
If you use a close-up too soon, you may gain immediate intimacy, but lose a great deal of information conveyed through body language that may be part of the actors' performances.
Too much use of close-ups also produces "Talking Heads" type of movies, a TV show disease.
The close-up should be a natural cut from a medium shot that is technically unnoticeable, but emotionally poignant or revealing. It should be a subtle enlarging of the presentation and used sparingly like salt.
There are all kinds of close-ups, some are extreme, showing eyes or lips, others are soft and romantic; however, all close-ups should be well lit and photographed with artistry, such as, when appropriate, back-lighting or using special filters and nets, since they are usually the easiest to control and much of the photographic excellence of a movie is judged by them.
Also, it's what actors are most concerned with. So pay a lot of attention to your close-ups. Much of your popularity as a film maker will depend on how you make your actors look. They'll literally love you for it! I mean that only figuratively of course!