Robin Williams' X-rated prank during filming

Robin Williams as Mrs Doubtfire. Photo / Getty Images

When you think of childhood movie classics, it's hard to go past Mrs Doubtfire.

The film had everything a good 1990s movie needed — Matilda star Mara Wilson, a cameo from the Olsen twins, Sally Field's excellent facial expressions and the genius of Robin Williams, who sadly died in 2014.

But while the 1993 movie might be a popular kids movie, what probably isn't well known is the very NSFW stunt Williams pulled while filming.

In a reddit ask me anything interview Williams did in 2013, the late actor revealed he decided to prank a sex shop while filming Mrs Doubtfire.

Why? Because he could, that's why.

"One time in makeup as Mrs Doubtfire, I walked into a sex shop in San Francisco and tried to buy a double-headed dildo," Williams said.

"Just because. Why not? And the guy was about to sell it to me until he realised it was me — Robin Williams — not an older Scottish woman coming in to look for a very large dildo and a jar of lube.

"He just laughed and said, 'what are you doing here?' and I left. Did I make the purchase? No. Did I walk away with a really good story? Yes."

As the movie celebrates its 25th anniversary, here are some other things you probably didn't know about Mrs Doubtfire.

WILLIAMS WAS 'ABSOLUTELY EXHAUSTING' ON SET

In an interview with The Project in October, Field described filming with Williams as "insane" but "so fun".

"He had endless energy," Field said. "And he'd want to do take after take after take because every take he would want to do something different.

"But what drove him absolutely crazy is that he could never make me laugh. He would never break me up."

MRS DOUBTFIRE'S BIZARRE AUDITION PROCESS

Matthew Lawrence, who played Matthew in the film, revealed his audition for the role was nothing short of strange.

Lawrence, who was 12 when he landed the role, told Page Six he was flown to San Francisco to audition for the role.

"There's this couch in the middle of a little stage area and they sit me down on the couch and all of a sudden this elderly British woman sits next to me," he said.

Lawrence said the woman began wrestling with him and his reaction drew laughter from those watching.

After the bizarre fight, Lawrence discovered the woman had in fact been Williams in his Mrs Doubtfire costume.

"Even in person, it was so believable," Lawrence, who's been acting since he was four years old, explained. "I have yet to see an actor pull off something so extreme, so well in person, not just on film."

THE MOVIE WAS SUPPOSED TO BE A LOVE STORY

According to IMDB, Mrs Doubtfire's scriptwriter was fired after writing an ending where the parents didn't end up back together.

But after rewrites the producers decided the original ending was better because it was more realistic and they rehired the scriptwriter.