Improvisational comedy is one of the major pillars involved in comedy as a whole, and it has a very rich history that dates back to the Vaudeville period and the 30s and 40s under theater teacher Viola Spolin. The system functions off of the principle of "Yes, and..." which means that players makeup everything off of the top of their heads, agree with the base reality of a scene, and add something new to the action unfolding in front of the audience's eyes.
The system is used in ways that aren't comedic as well, but even something as serious as a breakup scene tends to get silly very quickly. This format exists in almost every big-budget comedy to some degree, but there are plenty of comedians who make it their primary art.
Dan Akroyd
Dan Akroyd is a major player in a lot of productions spanning decades of comedy. From the run he had on Saturday Night Live to being the mind behind Ghostbusters, there's really nothing in the world of comedy he hasn't done.
Dan Akroyd started his career in comedy at The Second City in Toronto, a world-famous improvisational comedy institution. He co-founded the House Of Blues, was also in a film based on SNL characters called Driving Miss Daisy.
Michael Cera
Michael Cera is known for his incredibly awkward persona, which has pretty much been the main word used to describe his characters in every production he's been in ever. Michael Cera radiates uncomfortable vibes, but in an incredibly charming way that was meticulously crafted while working with improv troupes in his younger days.
He appeared in a few commercials as a child, but only developed a ion for acting after becoming obsessed with Ghostbusters and idolizing Bill Murray, which inspired him to go to The Second City, where Murray had also trained.
Steve Carell
The Office. Steve Carell has also shown up in a lot of serious roles as well over the years, appearing in films like The Big Short and Beautiful Boy, a drama about addiction.
As of late, it seems Carell has returned mostly to comedic roles, like the new television show Space Force. He also mentored Stephen Colbert during their tenure at The Second City.
Tina Fey
Tina Fey is an absolute titan of comedy, who also studied at The Second City in addition to Improv Olympics. After having worked on the house team at Second City for quite some time, Adam Mckay, the head writer for SNL at the time told her that she should send in some scripts and see if maybe the executive producer Lorne Michaels would like to include the sketches.
He liked what he saw and decided to have her on as a writer, and when Adam Mckay stepped down, she was made the first woman head writer for the show. She's since starred in tons of comedies, in addition to writing Mean Girls.
Bill Murray
Lost In Translation. He too got his start in improvisational comedy. He was recruited to Second City by his older brother Brian Murphy, after which he was recruited by John Belushi to a National Lampoon radio show.
He replaced Chevy Chase on Saturday Night Live for three seasons before finding a creative partner who he would work with repeatedly in Harold Ramis, who has a film school named after him under the Second City umbrella.
Mike Myers
Mike Myers is famous for Wayne's World, which was followed up strongly by the Austin Powers franchise shortly after.
He appeared in commercials as young as 2 years old, but his career as a child actor didn't take off in any major way. Perhaps the start of his journey was his acceptance as a touring member of The Second City, an improv school he studied at in addition to another called Improv Olympics, which has recently closed.
Amy Poehler
Amy Poehler is a founding member of one of the greatest improv groups of all time, The Upright Citizens Brigade, also known as UCB. She was on this team with Matt Besser, Horatio Sanz, Ian Roberts, Matt Walsh, Adam Mckay, Rick Roman, and Drew Franklin.
Three of the wrote The Upright Citizens Brigade Comedy Improvisation Manual, which is pretty much the most important improv manual on the market right now. She started at Second City and Improv Olympics, before Horton Hears A Who!.
Paul Reubens
Paul Reubens is perhaps better known by the name of a character that he cultivated during his tenure at The Second City. This character goes by the name of Pee-Wee's Big Adventure.
Pee-Wee's Playhouse was an absurdist/surrealist children's program, although it sometimes bordered on a show with a little bit more innuendo than something that would generally be considered okay for kids. Special guests were frequently featured, and very often, those were friends of Paul's from his improv days.
Will Ferrell
Will Ferrell pretty much needs no introduction, and one of the best things about improv is that plenty of people have started well into middle age or later in life, Will Ferrell being a great example of this. Ferrell went through a couple of unsuccessful careers before landing in comedy after being encouraged to pursue it by his mother, including being a mall Santa and a valet.
Eventually, after taking classes at The Groundlings Theater in L.A. he ended up making the house team, after which he was recruited for a 7-year tenure on Saturday Night Live.
Kristen Wiig
Kristen Wiig has had an incredible run on Saturday Night Live which she landed after her manager at The Groundlings suggested she send in an audition tape. She very nearly didn't get into acting at all, instead of majoring in art and being lined up for a job drawing portraits of people after their plastic surgery.
Instead of going to work to do that, she decided she was moving to Los Angeles and immediately fell in love with improv comedy. She had a 7-year stint on Saturday Night Live in addition to starring in tons of comedy films.
