Robin Williams was a very special case. His mind was always on the go. Most of the Genie’s dialogue from Aladdin was just Williams’s improve. He was such a master of it.
Yet when he got serious in interviews, you could hear the tears of the clown.
ETA: I’ve talked to both John Travolta and Kelsey Grammar in person. Not only are they not funny, they are both dumber than a bag of rocks.
My wife and I saw Tim Allen a few months ago. The non political stuff was okay, a bit dated. The political stuff was not funny to many. How many funny Obama jokes are there? Not many in a area that is very liberal.
Steve Martin eventually decided to short-circuit the whole process, handing out a signed card that explains how you’ve met him and he was funny – as well as warm and polite and intelligent.
Jonathan Winters was exactly the same way. JW was one of Robin’s main inspirations and they bonded like brothers when they met. Robin made Jonathan’s character on Mork and Mindy when he heard that Jonathan was out of work.
Maybe he steals his material
Yeah, or an IT-type. Lucky for me, people don’t tend to ask engineers to design stuff for them at a cocktail party. 
This explains the typical case. Comedians have routines that they’ve practiced and refined before audiences. Even the seeming adlibs in a routine may actually be scripted. Off-stage, in random circumstances, with no warm up, with an audience that may not have been drinking or expecting humor a comic may be in unfamiliar territory. On top of that, a lot of people don’t like to be asked to work in a social situation, already mentioned were doctors and mechanics, I’m frequently asked to fix peoples problems on their PCs as if I know the intimate details of each of the 10 gazillion pieces of software they could be having a problem with.
I know that many comedians explain the origin of their comedy as a way to deal with childhood awkwardness, bullying or isolation. Comedy was a way to defuse conflicts and be seen in a more positive light by their peers.
In that sense, then, comedy has always been an act for them and once they’re successful, it’s no longer a necessary defense in their personal life. At that point, they can be “real.”
But even if someone is naturally funny in real life, you don’t expect them to be funny all of the time - or even most of the time. The expectation that a comedian would be funny all the time doesn’t really make sense.
A good friend of mine is a touring stand-up comedian. He long ago stopped telling people what he did because the response was always one of three things.
QFT
I saw a clip on a commercial for some movie where he was running down the street naked, but he wasn’t being chased by a mob with hot tar and feathers.
It ruined the Suspension of Disbelief.
(It screwed up the alignment too).
I remember watching a light hearted interview show here in the UK in which Harrison Ford appeared. I can confirm that small talk did seem to be a struggle for him. I would say this was also the case with Gene Wilder. I can’t remember which show he appeared on but Wilder’s interview was one of the most boring interviews I have ever witnessed.
See, this is funny. I’ll bet if you stood near the good Count B. at a party, you’d hear a lot of witty repartee whispered to whoever was near. Now, I’m betting he isn’t a pro comedian, but I’d say he’s funnier. I’d much rather hear off-the-cuff witticisms than a routine.
Yeah I would say it’s more like an issue of persona or mood than anything.
I’m not sure what it means to say someone is not funny…no-one would describe themselves that way, because we can all make people laugh who have a similar sense of humour to us, and when we’re in the mood.
Groucho Marx was said to have been dour at times, when not performing, and it’s not too hard to find passages in his autobiography that bear that out. Still, I don’t think you could ever have called him unfunny.
Nevertheless, I’m sure I’ve read somewhere that it was either Zeppo or Gummo who was the funniest Marx brother in ordinary social settings. And that wasn’t just something people vaguely remembered from the early days when one or the other of the two youngest Marxes was a member of the act. Both of them also moved out to California, and they seem to have remained close all their lives.
I’ve heard Lucille Ball was as bad impromptu as she was good with rehearsed material. And she was great with rehearsed material.
She was a comic actress, not a stand up comic. The same can apply to a lot of actors mentioned here. Some of them would do just as well in a stand-up environment, or just ad-libbing, others simply need a script to work from.
Groucho Marx was mentioned above, many don’t consider him to be the funniest of the brothers, just the one with the best role. I recall some comment that Harpo was much funnier and Groucho pushed him into the silent role to keep him from being competition for the top banana spot.
Aziz Ansari has a great bit about being at a party and being asked to tell a “New Year’s joke”. The whole point of standup is to painstakingly craft the illusion of off-the-cuff wit, so it’s understandable when people think instant jokes are easy for comedians.
I spent a few minutes alone with Julia Roberts once. At first we just sat in silence, me thinking she wouldn’t want to be bothered chit-chatting with me and her, as I learned later, rather shy. As soon as I got uncomfortable with the silence and began a conversation, she seemed genuinely pleased that I’d taken on that burden and replied.
During their first trip to the US, at a press conference a reporter shouted to the Beatles “Sing us a song.” I think it was Lennon who replied “You have to pay us first.”
Comedic actors work from a script written for them. Standups work out their material before they tell the jokes. And they don’t necessarily do all of their writing either.
This thread reminds me of a bit from a 30 Rock episode where Liz was reminiscing how bad Jenna Mulrooney was at improv. In the cutaway scene, Liz and Jenna were shown on stage, ready to act out Billy Bob Thornton and Oprah on a subway. Liz said in a gravelly voice, “I shore like them french fried pataters,” and Jenna replied, “No you don’t, Oprah.”
I’m sure any improv artist would have wanted to strangle Jenna on the spot.