It depends on your doctor and on whether your doctor accepts assignment.

If your doctor accepts assignment:

He or she can only ask you to pay the 20 percent coinsurance (35 percent for mental health services) up front (and your Part B deductible if you have not yet reached it—$147 in 2013). Your doctor files the claims, and Medicare pays the doctor directly.

If your doctor does NOT accept assignment:

Your doctor may ask you to pay the full amount for services in advance and charge you up to 15 percent more than Medicare’s approved amount under federal law. Some states have stricter limits on what your doctor can charge you.

Medicare will reimburse you directly for its part of the bill (80 percent of Medicare's approved amount for most medical services; 65 percent for mental health services).