Walking interviewers through your thought process may help, as will structuring your answers.
For example, if they ask you 'why commercial law', start with building on what first sparked your interest in law (something you read, an event, a conversation, work ex etc.)

talk about the initiatives you took to explore this interest further

end with how your experiences and initiatives helped you understand that your skills and abilities are well-suited to a career in commercial law (eg- so if you went to an Open Day where you participated in a negotiation exercise, talk about which parts of the exercise you enjoyed and which parts best utilised your skills and how this helped you understand that you were fit for a career in commercial law).
Using a solid structure will help you fill in details and presents more thoughtful answers. I hope this helps!