You may have noticed a credit in movies for Foley artists. These people produce sound effects using real objects, movements, etc. that simulate a real sound in a movie. For example, if someone is zipping up their hoodie in a movie, a Foley artist may be called on in post-production to make that sound more audible on the sound track - usually by simply placing a microphone next to a zipper that can be worked with and recorded more easily than an actor on set. Foley artists may make the crunching sound of someone walking in snow, or an animal supposedly scratching on a door to get in or out, or the screech of fingernails on a blackboard.
A Foley artist's primary function is not to call attention to what they're doing but to make sure the audience isn't distracted by what's missing in the soundscape of what's being seen. So, when a director watching a scene realizes that something is missing from the soundscape, that sound must be added to what's already there.
It's easy for sounds to be missing when shooting live action. Microphones are typically located in a shot off camera so as not to be seen, but also to optimize the pickup of dialog between the actors. Other sounds may occur in the scene, but the mics may not be able to "hear" them. Foley artists are usually assigned the task of filling in these sounds. Check out this rather unusual example on the files page
Animators face this issue all the time. Visual elements are almost never recorded in sync with audio elements. Dialog is usually the first thing recorded, even before scenes are shot. On the other hand, sound effects are almost always added in post-production, after scenes are shot. This makes sense. Dialog often determines dramatic action (e.g. facial expressions, body mannerisms, lip syncing, etc.) while sound effects are timed to precede action, accompany action, or follow action. So, in the first case, the action follows the sound, while in the other cases, the sounds depend on the action.
They key to good sound is that it must not be a distraction. It must enhance what is being seen only enough to not be missed. It mustn't become the unintended focus for the audience, but it must be present at precisely the right time and in the right amount. Notice, I didn't say it had to be precisely realistic. It simply has to help evoke the mental imagery you intend. Too much is as bad as too little. Finding the balance is what makes film making an art. Check out the video to the right.