The actor made his debut as the boy wizard's nemesis in 2005's Goblet of Fire. In the final film, released next week, Voldemort takes on Harry (Daniel Radcliffe) for the last time.
Speaking at the Harry Potter UK press conference in London yesterday, Fiennes admitted that he found his costume "irritating".
"It was too long and sometimes I would trip over it," he said. "Underneath I would start wearing tights and the gusset kept dropping down between my thighs and this made it very difficult to walk with any kind of dignity.
"So eventually I said to my dresser, 'You're going to have to cut these tights and I'm going to have garters at the top'. So I enjoyed having this garter belt on each leg."
Fiennes quipped: "Sometimes when the stunt team were getting too macho I would tease them with my inner thigh."
Harry Potter producer David Heyman then declared: "People will never look at Voldemort the same way."
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Additional reporting by Simon Reynolds.