Hi!
I've of course never interviewed anyone for a TC, but have been on interview panels in a professional context & done lots of mock interviewing (specifically for Oxbridge Law applicants) and think this can be really common so wanted to throw my thoughts in. For what it's worth, I think it can often be a natural consequence of two things: i) nerves & worry about the need to impress, ii) the idea that you need to present yourself as 'like a lawyer' and all the (often outdated, stereotypical) connotations that come with that.
Naturally, you'll always be nervous and a little nerves are a good thing to focus you, but I think the latter (thinking for example Lawyers need to sound like X, use words like Y or give answers like Z) can be overcome and I often find with candidates it's about having the confidence in yourself and what you bring to the table.
The best bit of advice I think I ever got in relation to this and so now often give out is remembering that the people interviewing you are looking to see if you are a person who they want to work with; because of your potential to be a great solicitor, and the personality you are bringing to the firm (or for Oxbridge, looking to see if you are someone they want to teach). So ultimately 'you', the real 'you' is the best person to present in that interview. This doesn't mean treating it informally, but knowing that you are more than talented and professional enough to be able to present the person you would be at work and if that's not right for that firm then there's a better one out there for you, without having to worry about whether what you're saying is what a lawyer would say, which I think can often work to hide the genuine enthusiasm candidates have.
Keeping this in my head allowed me to answer questions in the way I genuinely would if I was asked a question at work, treating the interview like a conversation and naturally then relaxing into what was happening, which allowed me to show the enthusiasm I had & relaxes your interviewers themselves. I appreciate it's often easier said than done, but if you can take that confidence that you are a great candidate into the interview with you, then it makes it much easier to demonstrate your enthusiasm because you're not worrying about the other things.
I appreciate this is a long post so tl;dr: Enthusiasm & energy in interviews are often linked to confidence. Relaxing and being your true self can make it much easier for you to treat it like a conversation and properly demonstrate your genuine enthusiasm.
Whilst this of course is massively personal & subjective and I might've completely misunderstood the point of your post, hope that's helpful, and apologies if it's not!
