When considering a piece of theatre to reflect on in your live theatre written question (a part of Component 1), it is worth considering the following:
While questions offering you a chance to write about technical elements will be asked you will almost always have the opportunity to reflect on the performance of a particular actor. Moreover, there is no guarantee that the technique you have prepared (such as sound design) will be asked. So, while, of course, you can and should prepare an answer on a technical skill you should also have answers ready in regards to performance. At A Level you will be expected to comment on how a performer uses different techniques to achieve a specfic goal, such as how physical performance techniques were used to convey emotion.
You should reflect on perfomances which show a wide range of ambitions skills in both the tehcnical and performance arts which you are ready to recount (having made notes at the performance.)
It doesn't hurt, also, to write about a play which you are familiar with. Matching techniques and skill with specific quotes from the play will demonstrate a good knowledge of live theatre from script to performance.