One of the significant problems with running a plaintiff’s practice is managing cash flow. Any fees earned are usually contingent fees, and cases are not always resolved in such a way and a such a time to meet professional and personal financial obligations.
There are at least four different ways to address this issue. First, you can visit your friendly banker and arrange a line of credit that you dip into went you need to meet payroll, significant expert expenses, etc. This is probably the most common way of handling the situation.
Second, you can do business with a company that provides funding for plaintiff’s lawyers. Here is an article describing what these companies do and what they charge. There are several different models out there and the interest rates for each of them are much higher than you would expect to pay a bank.