Third 'Night at the Museum' film marks Robin Williams' final appearance

Stiller does double duty, also appearing as museum caveman Laaa who embarks on an improbable romance with Wilson's lonely British security guard.

"He set a really high bar," said Stiller of Williams, paying tribute to the actor's kindness and generosity of spirit. In the course of making three films together, he said he never got over being a Williams fan, a sentiment echoed by director Shawn Levy.

And when Stiller's character and Williams as Roosevelt bid farewell to each other toward the film's end, it strikes a poignant note.

"This character (Roosevelt) has always stood firmly at the soul of this franchise," said Levy, who helmed all three films.

He was vague about the possibility of a fourth film.

"This movie is about letting go. It brings a certain closure for these characters," he told a news conference.

But given the realities of the business of film making, he added "I suppose I can't predict whether that resolution will change."

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'Night at the Museum' premieres in London