When and where should we use scene transitions?
In the Beginning
Every time one scene ends and a new one begins, some form of transition is needed.
Chapter headings and scene breaks within chapters will clue the reader in that some kind of shift is happening, but the narrative needs to reveal that as well. Especially these days, with the rise in popularity of audiobooks, the actual text of the story needs to reveal that this change is taking place. Readers who are listening to audiobooks won't see the blank line or symbol that acts as a scene break.
A scene's or chapter's opening transition needs to identify place, time, and viewpoint character.
Endings
Though most transitional information is usually placed at the beginning of a new scene, the ending of a previous scene can also hint at an upcoming change.
New chapters often require stronger transitions. However, sometimes a chapter ends with a cliffhanger, and the events of the scene continue in the next chapter. In this case, the transition at the beginning of the next chapter can be more brief. However, it still needs to reconnect readers with the viewpoint character, location, and event in case readers have taken a break between chapters and returned to the book a while later.
Combined Transitions
Sometimes, transitions join forces. One of the most powerful transitional techniques is to combine the ending of one scene with the beginning transition of another.
When a writer ends a scene with a teaser of what is to come, the scene in which that hinted-at event occurs may not need as much detail in its opening lines. At the very least, this teaser ending and the opening transition of the next scene will grab readers and pull them into that anticipated event.
This technique can also be very effective if you hint at an event in one scene and that event doesn't take place until later, whether you place a single scene between those two or several. Maybe you're switching to a new viewpoint character and showing what's going on with that person, between hinting at the event that's going to happen with the first character and actually paying that event off for the reader.
Mid-Scene Transitions
Minor transitions may even need to occur during a scene. (This is the unexpected transition I mentioned at the beginning of the post.)
Sometimes, a character's movement to a new location happens onstage, so to speak. The reader has a sense of moving with the viewpoint character, so no scene break is needed, because no time is really skipped while the character is switching locations.
For instance, the character might be in the house for the first part of the scene and walk outside for the ending of it. You wouldn't need a scene break to show this happening, because the reader can walk through that door and down the steps into the yard with the character.
These transitions may be only a couple of words or a few sentences. However, if no transition is included, the author may be guilty of what I like to call teleportation syndrome. The viewpoint character seems to just teleport, or magically appear, at the new location, and this change can feel jarring to the reader.