The answer to this question depends on the attorney and your case. Remember, for every case that proceeds into court, there are significant expenses before the trial ever begins.
These are expenses for things like deposition costs, filing fees, and investigator fees. Some attorneys may take your case on a contingency basis, taking a portion of your settlement if you win, but charging you nothing if you lose your case. Attorneys will usually only take a case on a contingency basis, though, if the attorney believes the case will win and has enough money of his or her own to carry the case through trial.
Some attorneys will ask for a certain amount of money up front, and then deduct their expenses from that amount. However, this is an important question for you to ask when interviewing attorneys.