The introduction is your prime opportunity to vigorously demonstrate your understanding of the question you have been posed. A clear and concise, though deeply considered essay, needs to be firmly rooted in an introduction that sets up the solid argument that is to follow. It's your chance to wave your proverbial sign, to say to the examiner "Hey look, over here, I know exactly what I'm talking about!" It's all about grabbing their attention whilst making your intentions clear.
It's so crucial then, that you read the question at least 3 times before you even set out to write anything. Highlight, underline, circle, do whatever you have to do. Each time you read it, another possibility may arise and offer the basis for interrogation of its nuanced terms. Though you are pursuing your own solid argument (for example, you have decided Dr Frankenstein is transgressive), a good argument will counter this with other possibilities, whilst ultimately proving itself right.(It is important to remember that offering other evidence does not weaken your argument, but can actually STRENGTHEN it.)
So, the question you have been asked is "To what extent is Dr Frankenstein transgressive?" Interrogate every. single. word. What does it mean, "to what extent?" You are obviously going to have to, at some point, reach a judgement (based solidly in evidence), that suggests what degree he is one. A balanced though forceful argument, the question suggests, will need to ensue. Secondly, what does it mean to "transgress?" In your introduction you are going to have to make it incredibly clear that you understand what this word means, or potentially might mean depending on varying introductions. If you wrongly define a word in the question, you could seriously lose marks, because your introduction is the foundation of your essay, and every paragraph should link to it.
You need to lay out the terms of your argument, even if this initially means putting "In this essay I am going to argue that yes, Frankenstein is indeed a morally trangressive character." To make this an argument, it could be altered to say "Though much evidence supports the contention that Frankenstein reaches the extremes he does out of sheer ignorance and chance, meaning he may not be accountable in the way a transgressor is, I believe that his actions suggest that he is aware he is pushing boundaries he knows he should not be", or something along those fancy lines. Hopefully you can see how, by introducing an argument in the introduction itself, you have not only given a taste of what is to come but given a direction, an opinion, that you hope to pursue throughout. A good introduction will keep your mind focused as you work through the essay, and if it is clear, referring back to it won't only help the examiner know exactly what you're talking about, but will keep you completely on track.
Don't ramble on too much, but don't worry if you go on for longer than you expected. Just do whatever it takes to demonstrate that you are taking the question very seriously, are considering it from multiple angles, that you are ready to delve deep into the question in the essay to come, and that YOU MEAN BUSINESS.