Orson Welles Should've Been In...

[Steve of Bloomington]

In 'Pre' as Pre

[Kris K]

Two suggestions that are flights of fancy: Jaws as Quint and All That Jazz as Joe Gideon.

[Greggulator]

It's not later period but Orson in place of Fred MacMurray in The Apartment...

[dave from knoxville]

It's not later period but Orson in place of Fred MacMurray in The Apartment...

Or as Uncle Charlie in My Three Sons

[Vic]

what about Orson as one of the faces in the video for 'Cry' by Godley & Creme?

Godley and Creme - Cry

[cavorting with nudists]

"Goose" in Top Gun.

[Wes]

Orson Welles IS...

Mean Mr. Mustard from the Sgt. Pepper's movie

Welles also would have been perfect as either Marvin Sunk, The Sun King (played in the actual film by Alice Cooper) or in Donald Pleasence's spot as reord sleaze merchant B.D. Brockhurst, which would have given the world the Orson Welles/Stargard musical number it never knew it needed.

Kenny Rogers' role in "Six Pack."  A county singing racecar driver being hectored by Erin Grey and a whole mess of children.

For two hours.

This might be the best answer. All due respect to the legendary Kenny Rogers, but the dialogue below was meant to be delivered by Orson Welles:

Brewster Baker: He's right, Swifty. Be polite, Mr. Logan here's a very sensitive man.
[takes a few steps away and looks at Terk]
Brewster Baker: On second thought, I got a good mind to turn this little kid loose and let him just whip your ass, Terk!

[Gregory]

Tom mentioned it right before the topic, but Orson Welles would've been great in Rock N Roll High School.
I'm picturing him as Mr. McGree, the music teacher, or possibly Screamin' Steve Stevens.

[Eric Fishlegs]

GHOSTBUSTERS as Slimer (or the ghost who be called Slimer in the cartoon if you want to be specific.)

[dave from knoxville]

Orson Welles IS Uncle Buck

[JBillington]

I'm watching Get To Know Your Rabbit, and guess who just popped up from behind a screen. I howled with laughter. It is certainly funnier than any of the 'jokes' in the movie thus far.

[Steve of Bloomington]

Chauncey Gardiner in 'Being There'.

[Sashamak]

Ghostbusters as Slimer.

[cavorting with nudists]

Nic Cage's role in Face Off.

[cavorting with nudists]

Actually, maybe Nic Cage's role in anything!