Sometimes, beyond expert direction, brilliant acting, and phenomenal writing, it's the unscripted, improvised performances that make a scene memorable. 

Here are some of the most iconic moments on TV that were actually, completely unscripted: 

1. Marshall's reaction to his father's death in HIMYM

Easily one of the saddest moments on the show is when Lily informs Marshall that his father has passed away. 

However, Marshall's heartbreaking response, "I'm not ready for this" was actually improvised because Jason Segal was not aware that his character's father was going to die. 

The last line in the scene was something Jason came up with, in the moment. He said, “I’m not ready for this.” None of us were.

                    - Writer Carter Bays to EW

2. Michael kissing Oscar in The Office

In the episode Gay Witch Hunt, Steve Carell was supposed to kiss Oscar Nunez only on the cheek, but at the last moment, Carell turned it into a kiss on the lips - and that's the shot that made it to the final cut. 

Oscar and Michael
Source: Screen Rant
I was thinking, "I can't believe he (Steve) did it." And then I was thinking, right after that, "Of course he did it!" 

                    - Oscar Nunez to IGN

3. Rachel's comment about her marriage to Ross in Vegas being 'the worst hangover' from F.R.I.E.N.D.S. 

It's no secret that Mathew Perry was the show's resident ad-libber. But, one of the most iconic lines from the show was actually improvised by Jennifer Aniston.

Rachel and Chandler
Source: We Heart It

In the episode The One After Vegas, Rachel responds to Ross' comments about their Vegas wedding being an actual marriage by calling it the 'world's worst hangover'. That line was improvised, because, the original line actually read, "Stop saying the word marriage!". 

Ross and Rachel
Source: Twitter

4. Sherlock apologizing for his 'thing' in Sherlock. 

Sherlock getting drunk and forgetting words, like us normal humans, is not something you'd expect to see regularly. 

This is exactly why Sherlock's comment, "Apologies on behalf of my…thing" from the episode The Sign Of Three became so memorable. But, at the time of the shoot, Benedict Cumberbatch actually forgot his line, and that's how the iconic dialogue came to be. 

Source: Reddit

5. Andy diagnoses that Leslie has 'Network Connectivity Problems' in Parks and Recreation. 

Chriss Pratt is a pro at improv which explains why one of his funniest lines from the show was improvised. 

Parks and Recreation
Source: Reddit

When Leslie is suffering from the flu, Andy searches her symptoms (been there!) and diagnoses it as "network connectivity problems''

Parks and Recreation
Source: slowrobot

6. Murray's double-entendre conversation with Jonathan and Nancy in Stranger Things. 

Remember when Nancy and Jonathan crashed at Murray's house, and the breakfast turned into a double-entendre conversation?

Stranger Things
Source: TV Time

Yes, that awkward-as-hell conversation was actually partially improvised. If you see closely, Nancy (Natalia Dyer) can't actually control her laughter in the scene!

7. The flashback scene where Rue fights with her mother in Euphoria. 

One of the most emotional scenes from the show Euphoria, Rue's drug-fight with her motherwas completely improvised by Zendaya and Nika, at their director's instructions. 

Euphoria
Source: shadow and act

The flashback drug-fight scene, where Rue fights with her mother, was only scripted as 'Rue and her mom have a fight.' But Zendaya and Nika completely nailed the scene, leaving everyone on the set deeply impacted. 

I walked into my office and I burst into tears, because whatever it was, Z and Nika had tapped into that real, ugly volatility of addiction — the violence and physicality of it — in a way that I hadn’t seen before onscreen. For seven minutes, it brought back too much.

                    - Sam Levinson to Refinery29

8. Moira Rose pronouncing baby as 'bebe' in Schitt's Creek. 

One of the most iconic things about Schitt's Creek is Moira Rose's extensive vocabulary, unique pronunciation, and fashionable wardrobe.

Moira Rose
Source: The Atlantic

However, her accent and pronunciation was not something that writers incorporated while developing the character. Rather that was all that Catherine brought to the character, including the iconic pronunciation of baby as 'bebe'. 

9. Nell's interruption of Theo and Shirley's conversation in the car in The Haunting Of The Hill House. 

One of the few jump scares on the show, technically this scene wasn't entirely unscripted since Nell was always supposed to interrupt Theo and Shirley's conversation. 

The Haunting of the Hill House
Source: Popsugar

However, on director Mike Flanagan's instructions, Victoria Pedretti 'jumped' into the conversation far earlier than scripted, because Flanagan wanted to capture Elizabeth and Kate's real reactions. 

Haunting of the hill house
Source: SYFY
I just told Victoria before we read the first take to come in substantially earlier than the written cue. The girls had another half page of dialogue to go through as written, and so Elizabeth and Kate were just in the scene and they knew they had another half page of words to get through before Victoria would appear, and she just bolted up right in the middle of their lines, and so their reaction is completely genuine. It also scared the s*** out of all of us at the monitor.

                    - Mike Flanagan to EW

10. Brienne & Tormund's relationship in Game of Thrones. 

Yes, the relationship that everyone was shipping, was actually not in the script. The way that Tormund stares at Brienne with evident interest causing her to bewilderely look away, was actually improvised by the actors. 

Brienne and Tormund
Source: Medium
There was the episode where they’re all leaving Castle Black together and there’s a shot — it wasn’t scripted at all — of two of them on horseback and she looks at him and he smiles at her. It’s not something you could ever write.

                    - Weiss to EW

The directors were so impressed by their performance, that, in the last season, they let them improv the rest of their scenes too. 

Not from the script, but on the screen!