The single most important thing to keep in mind when interviewing at a law firm is to understand how the law firm will make money when they hire you.
It sounds shallow, but law firms exist to make money. Hiring you to sit in an office and wait for work to land on your desk is a horrible strategy. Make the decision to hire you an easy one for the firm.
Will you bring clients? Will you solve a problem (i.e., handle existing work that is not being done)? Will you fill some internal hole in the law firm’s legal team that will assist the firm attract or support clients? Whatever it is, being able to understand and articulate the business case for hiring you is the first step.
The next step is to be able to articulate why YOUR solution is better than all other solutions.