It's been announced this week that James McAvoy and Claire Foy are to star in a remake of a French thriller movie in which McAvoy will improvise all his character's dialogue – quite the acting challenge, it has to be said. To whet your appetite and maybe even provide McAvoy with a bit of inspiration, here are 10 memorable movie moments that actors ad-libbed on the spot.

1. The Salty Pub Scene From Shaun of the Dead

In this hilarious scene from Edgar Wright and Simon Pegg's horror comedy, Shaun (Pegg) gets a cheering up from best mate Ed (Nick Frost) after he's dumped by his girlfriend. Frost improvised the rather outrageous stories that Ed tells about the pub regulars, and Pegg's thoroughly amused reactions are entirely genuine.

https://www.youtube.com/watch?v=l93fDmh6XKo

2. Pretty Woman's Jewelry Box Snap

This classic rom-com has lashings of charm which belies its dark scenario; in fact, it was originally conceived as a cautionary story about prostitution in Los Angeles. In this scene where rich businessman Edward (Richard Gere) shows escort Vivian (Julia Roberts) the dazzling necklace she'll be wearing for their night at the opera, Gere improvised the bit where Edward snaps the jewelry box shut, prompting a completely delightful reaction from Roberts.

https://www.youtube.com/watch?v=sNYalWdtPSA

3. The "Tears in the Rain" Monologue From Blade Runner

Arguably the ultimate cult movie, Ridley Scott's sci-fi masterpiece is packed with moments that fans pore over time and time again. Among the most memorable is the poignant monologue Rutger Hauer's self-aware android, Roy Batty, delivers as he realizes his demise. Remarkably, the version that made the movie was semi-improvised, with Hauer himself adding the most iconic line of all: "All those moments will be lost in time, like tears in rain."

https://www.youtube.com/watch?v=NoAzpa1x7jU

4. "You Talkin' to Me?" From Taxi Driver

In Martin Scorsese's 1976 thriller, Robert De Niro delivers one of his greatest performances as a lonely New York City taxi driver descending into madness. The movie's iconic mirror scene was improvised by De Niro from stage directions alone in the very last week of shooting. Scorsese recalled in 2016: "He kept saying, 'You talkin' to me?' He just kept repeating it, kept repeating it ... and the [assistant director] was banging on the door saying, 'Come on, we got to get out of here.' And I said, 'No, this is good, this is good. Give me another minute.' It was like a jazz riff. Just like a solo."

https://www.youtube.com/watch?v=-QWL-FwX4t4

5. The Orgasm Scene From When Harry Met Sally...

This fabulous scene shot at Katz's Delicatessen in New York City was a real team effort. Screenwriter Nora Ephron wrote the lines in which Meg Ryan's Sally shocks Billy Crystal's Harry by talking about women faking orgasms; it was Ryan's idea to fake one at a table in the middle of a restaurant; Crystal came-up with the much quoted pay-off line: "I'll have what she's having." And it was delivered by Estelle Reiner, mother of director Rob Reiner.

https://www.youtube.com/watch?v=b0OeM6UUAoI

6. Draco Malfoy's Witty Zinger From Harry Potter and the Chamber of Secrets

In this scene from the second Harry Potter movie, Daniel Radcliffe's title character uses Polyjuice Potion to disguise himself as dimwitted Gregory Goyle, one of Draco Malfoy's henchmen. When Draco asks "Gregory" why he is wearing spectacles, Harry replies that they're for reading, prompting Draco to fire back: "I didn’t know you could read." It's a line made all the funnier by the knowledge that actor Tom Felton improvised it on the spot.

https://www.youtube.com/watch?v=SP7Oe-BzplA

7. Spidey's Demise in Avengers: Infinity War

In the script, Tom Holland's webslinger was supposed to bid farewell to Robert Downey Jr.'s Iron Man by simply saying: "I'm sorry." But directors Anthony Russo and Joe Russo felt this didn't quite capture the full emotion of the scene, and duly encouraged Holland to come up with something more substantial. The result is a real tear-jerking Marvel moment.

https://www.youtube.com/watch?v=JVy-6GChSrA

8. “Here’s Looking at You, Kid" From Casablanca

Humphrey Bogart is said to have come up with this stone-cold classic line while teaching poker to co-star Ingrid Bergman between takes. Screenwriters Howard E Koch, Julius Epstein, and Philip Epstein liked it so much that they weaved it into the movie a further three times after Bogart first ad-libbed it in a take.

https://www.youtube.com/watch?v=dqhNZJKMFLc

9. Idris Elba's "Black Superman" Line in Hobbs & Shaw

Elba himself came up with this super-quotable line from last year's Fast & Furious spin-off after vetoing the suggested line – "I'm Black James Bond" – because he felt it was, well, too close to the bone!

https://www.youtube.com/watch?v=Z4s_J-PrA-s

10. Matt Damon's Charming Anecdote in Saving Private Ryan

Damon plays the title character in Steven Spielberg's Oscar-winning epic and adds emotional heft when he's finally found by his fellow soldiers, telling a sweet story about a night spent with his brothers before the war. Damon ad-libbed the whole thing in authentically rambling style, and Spielberg liked it so much he kept it in the movie.

https://www.youtube.com/watch?v=KJr-4OZPwLk

Do you have a favorite improvised movie moment?