Reading the entire file is a good idea. But there might be important undiscovered evidence that you should mull over with your client. A person receiving a bad deal, from either a business associate or state actor, is in shock and probably won't be able to identify specifically all of the important evidence. For example, search for injury claim relevant attitudes and personality movements, that reveal where the person is unable to speak, and find them to their words. The lawyer's job is also part therapist and part private investigator. The more time you take to care, the better your reputation as a good lawyer.
Sometimes you might receive a case where the file has grown extremely large. Then it might be best to point blank ask the client, where do you think the most wrong occurred, and then go search for it. Take notes.