Kevin Spacey with less hair?
You know, if you take away the Siskel part, James Coburn would fit.
Maybe we need two sets of ID matching; one for looks and one for voice. Balding tallish types in one stack and lean guys with deeper voices in the other. I’d suspect at least 50 in each category. We need us some face-recognition matching app for one part and a similar thing for the voices.
Have you ever noticed how often the really deep-voiced radio or voice-over actors are not all that tall and thin? Best offhand example I can think of: William Conrad, who, among other things was Matt Dillon on Gunsmoke on radio.
Al Traina?
He’s everywhere!
I always thought William Schallert (sorry no links, I’m on a tablet with no mouse) looked like Gene Siskel. What’s your time frame? if it’s something like 1962-1979, look up Schallert (he still works occasionally, but he’s a lot older than the age Siskel lived to).
Jeremy Piven?
Oh boy, talk about brain fart.
I though the OP said the guy looked like Gene Shalit. No wonder none of the mentioned choices made sense to me.
Carry on!
That’s way too funny! 
You won’t forget his face if he ever mugs you. But you might confuse him with his wife.
How about Bob Saget?
Maybe obscure but Richard Hunt who played Wilson in Trading Places seen @1:10 on
the video
Obscure Kenneth Utt?
My first thought was also JK Simmons.
So OP, you gonna come back to this thread?
For real! 
I’ll give it a guess, George Lowe from Space Ghost? (although he’s primarily a voice actor)…
Not that he has anything in common with Gene Siskel, and I can’t remember his voice all that well, but something about Michael Wincott made me look through a dozen or more of those “that guy” lists before finding him. At least his name is in my mind for however long it takes to slip out again. IMDb has oodles of such lists if you go looking for them. 
My apologies - I’ve been away from the web for a couple of days.
No hits so far, sorry to say. As far as I know this actor is still working. I’m terrible at guessing ages, but if I had to guess I’d say he’s in his late 40s or early 50s. Doesn’t play heavies, at least not in my memory. For some reason, I picture him as an attorney, or maybe a government staffer, possibly a not-terribly-important congressman.
Hmmm…maybe I should look through the IMDB page for The West Wing. Just about everyone was on there at one time or another, right? If not there, I might have to mine the Law & Order page
See you in a week. 
Yes – the guy I am thinking of is government bureaucrat. Although after looking through all the pictures, I can’t picture him anymore!
So, it looks like maybe I am thinking of John Bedford Lloyd (sorry - having trouble linking through TapaTalk). A few things:
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He doesn’t look nearly as much like Gene Siskel as he does in my head,
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He’s older than I thought (59), and,
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If you’re looking for “That Guy,” and you have the time, just scroll through The West Wing’s IMDB listing. Wow, everyone was on that show at one time or another.