Before his three-hour neck surgery for herniated disks in December, Peter Drier, 37, signed a pile of consent forms. A bank technology manager who had researched his insurance coverage, Mr. Drier was prepared when the bills started arriving: $56,000 from Lenox Hill Hospital in Manhattan, $4,300 from the anesthesiologist and even $133,000 from his orthopedist, who he knew would accept a fraction of that fee.

He was blindsided, though, by a bill of about $117,000 from an “assistant surgeon,” a Queens-based neurosurgeon whom Mr. Drier did not recall meeting.

“I thought I understood the risks,” Mr. Drier, who lives in New York City, said later. “But this was just so wrong — I had no choice and no negotiating power.”

In operating rooms and on hospital wards across the country, physicians and other health providers typically help one another in patient care. But in an increasingly common practice that some medical experts call drive-by doctoring, assistants, consultants and other hospital employees are charging patients or their insurers hefty fees. They may be called in when the need for them is questionable. And patients usually do not realize they have been involved or are charging until the bill arrives.