The element of surprise is what makes improvisational comedy so side-splitting funny, says Colin Mochrie.

"Improvisors don't know where the punch line is," says the comedian during a telephone interview. "We're as surprised as the audience is. Sometimes when you're watching a sit-com, you can tell where the joke's going to be before it happens. You can't really do that with improv.

"For me, standup is the hardest of all the comedy arts," he adds. "With improv, the audience knows we've got nothing. We're using their stuff, so they have a sort of a vested interest in us doing well. With standup there's more judgment. An audience will just sit back and think 'OK, funny guy, show us what you got.' "

Improvisors Mochrie and Brad Sherwood have worked together on and off for 28 years. They are fellow Whosers from the Emmy Award-winning hit TV show "Whose Line Is It Anyway" — "Where everything is made up and the points don't matter." More recently, they've been touring as a two-man group to deliver their unabashed humor and bizarre antics to audiences — such as "sideways scenes" or "lie-down comedy," when the comedians lie on the stage floor. Cameras shoot the scenes based on topics that include a shih tzu pooping on the lawn or Star Wars character Yoda returning a pair of hedge clippers.

Mochrie and Sherwood will perform at 7:30 p.m. Friday, May 19, at the Craterian Theater, 23 S. Central Ave., Medford. Tickets are $52, $55 or $58 and can be purchased online at craterian.org, at the box office, 16 S. Bartlett St., or by calling 541-779-3000.

Listen closely and you'll hear comedic references to parasitic fish, such as remora, a species with suction cups on their heads used to attach themselves to sharks, turtles, ships and other moving objects.

"Both Brad and I were going to be marine biologists when we were younger, so we try to throw biologist jokes at each other," Mochrie says. "It's fun for us, nobody else would care."

Mochrie attended Studio 58 in Vancouver, British Columbia, for four years, saw an improvisational comedy demonstration, took a couple of improv classes and discovered there was something about the art form that he took to right away.

"I loved the school because all the classes were taught by working actors," he says. "I not only got a chance to learn from their experiences but also hear about what it is actually like to be an actor, to be out there struggling to get parts. It was a good, non-sugared-coated version of what is was going to be like."

Mochrie found his first job at performing arts center Vancouver TheatreSports League, where he met improviser Ryan Stiles. He toured with the Chicago-based Second City comedy troupe until Stiles urged him to audition for the British version of TV show "Whose Line." He became a regular on the show until it ended in '88. He then joined the U.S. version on ABC, appearing in every episode. He continues on the CW Network's revival, showing on Charter Channel 11 or broadcast Channel 10.2.

"We're very fortunate to have "Whose Line" as a showcase," Mochrie says. "We all improvise. Our lives are totally improvised. We just take that aspect of our lives and put it on stage."

Mochrie's new book, "Not Quite the Classics," was published in 2013. Based on the improv game "First Line, Last Line," he takes the first and last line from classic literature and recasts the stories in between in his own offbeat style.

A verbose coyote becomes the star of "A Tale of Two Critters"; Ishmael is a struggling actor hunting for a good hair day in "Moby"; and Mighty Casey strikes out again in "Casey at the Bar."

"Yes, my agent made me write a book," Mochrie says. "It was hell. I don't know how people do it. It's just too much work. Part of the reason I'm an improvisor is because I'm lazy. I don't want to have to learn stuff. I just stand in front of people and make up crap with whoever I'm with. With writing, you gotta sit there, you gotta make sense, you gotta have outlines."

Mochrie's favorite improv games change all the time, he says.

"Especially when you're touring and you feel like you're getting into a rut. Then we come up with a new game that is exciting for us, so that's the game that is our favorite for a while. On 'Whose Line," 'Greatest Hits' was always my favorite. It gave Ryan and I the chance to banter and goof around, then hand the stuff over to incredible music improvisors on the show. It was a great game to be a participant and also an observer."