I nearly forgot about complications when it comes to adding dramatic tension to your book! But my ticking timer has reminded me of them.
Complications are things that happen that make a situation worse or -- no duh! -- more complicated. They can add to the conflict between the characters (I'll call that a character complication), or they can be separate from the characters (I'll call that a plot complication).
A character complication would be something like the arrival of Aunt Minny. On the surface, that may seem like no big whoop -- except that Aunt Minny knows all the family skeletons and is likely to reveal something the heroine would rather keep secret.
Plot complications are things that cannot be changed by any action on the part of the character. For instance, in the character complication of Aunt Minnie, the heroine could get Aunt Minnie to leave right away, so she doesn't have a chance to spill the beans. Or Aunt Minnie herself, seeing the situation, could decide to keep silent. In other words, a decision could be made that negates the complication, or makes it unimportant; a character can affect it. So it may be a major, life-altering complication, or it could just be a minor complication easily resolved after it's upped the tension for a little while -- preferably because something more important has come along.
(Note that character complications create more conflict. With this example, there's going to be conflict (a) between the heroine and Aunt Minny, especially if she doesn't want to leave or (b) within Aunt Minny, as she tries to decide to tell what she knows, or keep it to herself.)
With a plot complication, there is nothing a character can do. For instance, nobody can stop time (well, maybe in a paranormal, but I'm talking generally here). Nobody can prevent a tornado, or earthquake. They can only react to it.
Because they are uncontrollable, plot complications can really up the tension. How is the character going to achieve what he or she needs to accomplish if they cannot have any influence at all on what may prevent the achievement of their goal?
Now that's complicated.