to tremble with sudden fear: the involuntary shudder of a startled or terrified person, and (in causative form) to make someone tremble like that
At its heart this word names a physical reaction, not a settled emotion. It is the body's jolt when danger or the unexpected breaks in: the shudder that runs through Isaac when he learns he has blessed the wrong son, the start that jerks Boaz awake at midnight, the pounding heart of a king who sees the enemy army massed against him. Fear is in it, but the picture is always the tremor itself rather than the calm dread we might call worry.
Because the core image is a tremble that something causes, the word moves easily between the person who shakes and the thing that shakes them. On its own a person or a whole camp trembles; in its causative form someone or something makes others tremble, frightens them, or drives them off. The two are one idea seen from opposite ends, which is why a single word covers both.
The causative side gives the Old Testament one of its favorite pictures of peace. To live with no one to make you tremble is to rest without a single threat left that could startle you, the security God promises a faithful people. The same phrase turned toward judgment is chilling: a dead body left in the open with no one to frighten off the birds and beasts that come to feed.
One stretched use rounds out the range. When a woman is said to have trembled on someone's behalf, the sense has softened from fright into anxious care: she has put herself out, gone to trouble, worried herself over another's needs.