Be well aware of what you have put on your CV as well as anything else that you have done, and when you come in, don't fake it. be yourself
Be yourself, know your resume, put forward things that are interesting about you ie. your hobbies and interests.
Know yourself well and don't rehearse answers - just try to stay calm, genuine and in control
Understand the company, why they want to work there. Why commercial law. What makes the firm unique, why is that good.
Be comfortable with your own personal examples for behavioural questions, with responses (informally) structured
Know your resume well and know why (you think) you want to work here.
Try to relax, be yourself and don't over-prepare. People notice when you come across as contrived. You only want to be part of an organisation that wants you for you.
I would suggest candidates allow their individuality and personality shine through in their cover letter, as well as an honest desire to work at the particular firm as opposed to other law firms. I would recommend they practice mock interviews with a friend or colleague before hand.