Here's a tip from Ashley Lipson's Guerilla Discovery. To insure you're getting the full story from your client, try asking a question something like the following:
I know that you've told me the truth and I believe you. But we have to prepare for those lying S.O.B.'s on the other side. What kind of lies are they going to tell? Tell me what lying description of the events they are going to tell so I can prepare for them.
Lipson suggests this question for deposition preparation, but it might also be useful during initial client interviews. According to Lipson, it's likely you'll hear some things from your client you didn't know before--things you'll need to know about your client's case before getting in too deep.