This piece is not about the power and purpose of an effective introduction to the essay. Before moving on, I suggest reading this other piece first.
Here, I want to address one specific feature of a good introduction. In the pre-writing phase, building the introduction is limited to two construction tasks. And once you have locked in a provocative thesis and hammered out an arresting opening ('an attention getter'), there is nothing else to plan for in the introduction.
But that is only insofar as the planning phase is concerned.
In the writing phase, the introduction is achieved not simply by placing the chosen thesis next to the attention-getter. The transition from the attention-grabbing device (the quotation, story etc at the opening) to the thesis almost always requires a bridge.
A bridge is made up of sentences that move—and move swiftly—in the direction of the thesis. The function of the bridge is to establish the relevance of the attention-getter to the thesis.
If the bridge is too short, it will result in a choppy introduction. Even if the opening device is powerful, it can easily misfire, unless it is carefully escorted to the thesis. But care is not the enemy of progression. The bridge should not be longer than necessary. It should not drift casually towards the thesis. A strong bridge progresses steadily towards the thesis—robustly without rushing to its goal.
There are at least four distinct bridge-making techniques. To start with, the essayist may draw out an implication from the attention-getter: a lesson learnt, a corollary (to the striking fact, quotation etc that set the essay into motion). The implication should walk the attention-getter right up to the essay thesis.
Secondly, the essayist may draw an analogy between, say, the situation in the attention-grabbing story or quotation at the outset and a parallel scenario that prefigures and sets up the thesis.
A third option for the essayist is to raise a question based on the attention-grabbing device (the definition, the story etc). The question should be so constructed that it immediately requires the statement of the thesis for its answer.
Another way to create a smooth transition between the opening and the thesis could be to follow up the attention-getter by highlighting an aspect of it that anticipates the thesis. This may be a short comment on a point made by the opening device: the point in turn becomes the reason for bringing up the thesis.
To be clear, these are not the only techniques or options for bridge-making. Other factors being equal, a smooth introduction always leads the essay from the front.