For preparation, make sure you understand the role and responsibilities of trainee solicitors and their position within a law firm. Then, practice as much as you can, ideally with someone else asking you the questions (I'm sure you can find many practice questions online, they don't have to be law-related).
I often see students answering the question in a way they think will impress. e.g. they'd skip their brother's wedding and stay in the office to help a trainee. It comes across far far better if you're being thoughtful and genuine in your approach.
When you're asked the question, my advice is to pause and try to put yourself in the scenario. Really think: "What are the steps I would take in this situation?" and talk through the steps you'd take out loud, justifying your reasons as you go.
If it helps, here's some advice I gave a student a while back based on this scenario question:
You just started your training contract. Your supervisor has given you a task that needs to be presented to the client in 7 days time, you have been working on this particular task. You know you have gaps in your knowledge on that subject area and now you are extremely busy with other matters.
"I can tell you how I would answer the scenario, but remember, I answer in a particular way because I have been in a similar situation. Your answer to this question may well not be the same (and that is fine).
- Make sure I understand what is involved in the client presentation and try to estimate how much time it would take to prepare.
- Identify the gaps in my knowledge and determine what I need to do to get up to speed.
- Inform my supervisor of my situation just so that he or she is aware. Be open about the gaps in my knowledge and ask for suggestions on what to do.
- Make a note of the client presentation in my to-do-list.
- Determine which of the existing matters are urgent and non-urgent. See if any of them can be pushed back. I may need to speak to other people to find this out.
- If, after this, I have enough time to prepare for the presentation, then carve out that time in my diary. Research and/or speak to people to fill the gaps for the presentation.
- If my work won't clear up, or by day 2 or 3 I realise that I will not have enough time to prepare for the presentation, go back to my supervisor and explain the situation. Make it clear that I want to help, but I don't want to risk being unprepared. If another person is picked, I can suggest I bring the person up to speed on the topic as I have been working on the task. I can then carve out some time in my diary to explain what I have been working on and anyone that is worth speaking to. I can also help (time-permitting) the person prepare for the presentation."