Unfortunately, I’m going to ask you to help me based on not too much information. He’s tall and slender (or at least comes off that way), has a fairly deep voice, and his face reminds me a great deal of Gene Siskel.
And, that’s all I’ve got. Good luck!
Old movies or more recent ones?
Gregory Peck?
James Stewart?
Genre? Medium? Time frame? 
The Gene Siskel thing plus just having seen him in a decent movie made me think of Mark Strong.
I think I know who you mean – no, seriously. He plays kind of an everyman, not a real violent/heavy type guy? Of to review the IMDB list of character actors…
Was he that guy who was in that movie that time?
To me, Stanley Tucci looks a bit like Gene Siskel.
Lorry Goldman played “Gene”, Mayor Ebert’s aide in Godzilla (1998), based on his resemblance to Siskel.
Well, I looked through
http://bplusmovieblog.com/2011/09/01/in-appreciation-of-character-actors/
which had everyone but the guy I’m thinking of! But maybe you will find the one you’re looking for?
Elias Koteas is a poor man’s Mark Strong
Charles Martin Smith ? but he’s short…
Michael Gaston is vaguely Siskel-like.
F. Murray Abraham
J. K. Simmons
Not an answer, but I just recently watched thison Netflix and this thread reminded me of it.
He always reminded me of Gene Siskel a little.
This was going to be my answer, as well
As long as we’re still throwing out names while we’re waiting for the OP, how about Rex Harrison or Ralph Bellamy.
That’s him!
No, it can’t be him. He was in that other thing with that other guy. You know the one.
Paul Adelstein?