[Reaganesque]

There's quite a few to choose from but I would have to mention:

Billy Crystal and Carol Kane in "The Princess Bride"
Bye, bye boys! Have fun storming the castle!

Marcel Marceau (famous French mime) in Mel Brook's movie "Silent Movie." The movie is totally silent but for Marceau's response to Mel Brook's character's invitation to be in his new movie: "No."

[Henchster]

Gene Hackman as the blind man in Mel Brook's "Young Frankenstein."

"Wait! Where are you going? I was going to make expresso!"

[A CA Guy]

[AnnaZ]

Ann-Margrock and Cary Granite.

;^)

[Energy Alley]

Marshall McLuhan in Annie Hall, "you know nothing of my work"

[Revolting cat!]

Jon Voight's Chrysler Le Baron in Seinfeld.

[Charles Henrickson]

Suzanne Pleshette's cameo on the last episode of the "Newhart" show, when she and Bob wake up--as though they were still on the previous "Bob Newhart Show"--and Bob says he just dreamed the whole New England thing.

[MattinNJ]

The Transporter handing a package to Tom Cruise's character in the openening scene of Collateral.

[saganite]

William Shatner in Airplane 2.

[heights]

[Hildy]

The best...ever....SHIRLEY MCLAINE in DEFENDING YOUR LIFE in the HALL OF PAST LIVES. Just soda-spitting hilarious when she came on the screen.

[Centurion2000]

I think I'd have to say when Sean Connery stole the show from Kevin Costner in the Sheriff of Nottingham. .... And the bad guy was ssooo much better than Kevin in that film

[AnnaZ]

The Princess Bride scene you mentioned is also an all-time fave. (The paper cut line, the MLT...). Then again the whole movie is absolutely brilliant -- start to finish.

I really wish Rob Reiner would've stuck to movies.

[Mat_Helm]

John Wayne in "The Greatest Story Ever Told' as the Roman centurian standing at the foot of the cross and proclaiming, "Truly, this was the Son of God!"

[Lunatic Fringe]

Bruce Willis in "Friends":

"You.. are a neat guy. I'm just a love machine..."

[Cowboy Bob]

Susan Pleshette appearing in the final episode of the "Newhart."

[greene66]

In terms of tv, one I recall as being amusing came at the end of an old "I MARRIED JOAN" episode from the early-50s. The episode had Joan involved in placating some tempermental (opera?) stars, and during the tag at the end of the episode, someone knocks on the back door. It turns out to be Bing Crosy, introducing himself as a new neighbor, and wanting to borrow a cup of sugar or something. After he leaves, Joan disgustedly mutters, "well, there goes the neighborhood."

[kaboom]

George Clooney on "Southpark" as the voice for Stan's gay dog.

[andysandmikesmom]

There were two 'Invasion of the Body Snatchers' films...one made during the 1950s, the other made during the 1970s...the male lead in the 70s movie, was Donald Sutherland...the male lead in the 50s movie was Kevin McCarthy...of course the 70s version was a remake of the 50s version...

In the 70s version, folks are in a car, trying to get away..when all of a sudden a guys runs in front of the car, hollering something like "Run, They're coming"...that man running in front of the car is Kevin McCarthy....he had the lead role in the 50s film, and a cameo in the 70s film...

[HuntsvilleTxVeteran]

Glen Campbell .... Capt. Hazeltojn in "Uphill all the Way".

[steveo]

Arnold Stang, Marvin Kaplan in a "It's a mad mad mad mad world". Both were also cast in the cartoon series "Top Cat."

Stang voiced "Top Cat" and Kaplan voiced "Choo Choo".

An interesting trivia point, is that "Top Cat" was modeled after The Phil Silvers Show were he played MSgt Bilko. Silvers was also in IMMMMW appearing as 'Otto Meyer'.

Additionally, in the "Top Cat" series, the character "Bennie the Ball" was voiced by Maurice Gosfield, who was "Doberman" in the Phil Silvers show.