‘Fathers’ Day’ gets a boost from ‘Friends’ in a new twist on one of Hollywood’s favorite concepts.

In an unprecedented example of that most over-used Hollywood term, “synergy,” Thursday’s episode of “Friends” will double as a thinly veiled promotion for “Fathers’ Day,” the new movie starring Billy Crystal and Robin Williams.

Synergy, in this case, because both “Friends” and “Fathers’ Day” come from Warner Bros. through its television and feature-film arms, which coordinated and timed the cameo appearance to tie into the film’s opening on Friday.

While networks have frequently had characters from one series pop into another as a promotional tool, using a studio’s TV programs to tout one of its features represents a new twist on an old marketing gimmick.

Those involved suggest the Crystal-Williams pairing offered a “unique opportunity” to capitalize on two well-known comedy stars, in a scenario that benefited NBC as well as the studio and the program’s producers.

“Sometimes an idea comes up that just works for everybody,” said Nancy Kirkpatrick, Warner Bros. vice president of publicity.

Industry officials anticipate that there will be similar stunts in the future as studios seek to deploy their assets across divisional lines.

“I know in the next couple of years I’m going to be hearing about a lot more of these,” said Warner Bros. Television President Tony Jonas. “There’s a sharing that’s going on now [between divisions] that we’re taking more advantage of.”

In fact, the promotion is less grandiose than was originally envisioned. Warner Bros.’ publicity department (which came up with the idea during a brainstorming session) initially proposed having Williams and Crystal show up on each NBC program that the studio produces Thursday night, including “Suddenly Susan” and “ER,” similar to some show-hopping theme nights NBC has featured in the past.

“Susan,” however, had finished production for the season, while the actors’ schedules couldn’t be orchestrated to fit into “ER.”

The two will appear only during the teaser (that is, pre-credit) sequence on “Friends,” which, as one of prime time’s top-rated programs, reaches an audience of 25 million people during an average week, many in the teen and young-adult demographics that represent the primary moviegoing audience.

An NBC spokesman said that there is no overt tie-in to the movie in Thursday’s episode and that the network was obviously delighted to get such promotable stars to appear during a sweeps period.

Insiders note that such stunt castings remain subject to approval of the network in addition to the producers involved, meaning that Warner Bros. couldn’t push through a cameo that didn’t appeal to the network just to promote an upcoming feature.

Warner Bros. will also buy two commercials for the movie that will air in NBC’s Thursday lineup, but not during “Friends.”

“The whole idea is to keep it on the highest road possible,” Jonas said, acknowledging some internal concern that such stunts not be perceived as cheapening a series. “I’m sure the feature division as well as [the TV arm] will be very selective as to where it’s appropriate.”

Studios with network ties haven’t exactly been bashful about using prime time to promote their films. Both ABC and Fox, for example, have broadcast specials connected to the release of “The Hunchback of Notre Dame” and the “Mighty Morphin Power Rangers” movie from their studios.

Bruce Willis, who will be seen later this month on NBC’s “Mad About You,” also headlines “The Fifth Element,” which opens this weekend. Although both the movie and series are from Sony Entertainment units, a studio source said the guest role grew out of Willis’ friendship with “Mad” star Paul Reiser and is unrelated to the film.