Don’t know how to describe it, but some actors and probably actresses have a lot of physical intensity which is almost palpable when you watch them.
Klaus Kinski
Clancy Brown
Ron Perlman
Brad Dourif
Usually they play villians or twisted characters. Anyone have any idea what I’m describing in these actors or can you name others along this line?
I heard many people who had to work with Kinski on different films seriously considered murdering him.
Christopher Walken, of course.
I would add Michael Madsen and Tom Sizemore. They also fit the mold of “usually they play villians or twisted characters,” so you are spot on with that observation. And of course Samuel L.
Female examples are harder to come by, but Amanda Plummer would be one candidate. Can you tell I’m a Tarantino fan?..TRM
Christian Bale aims for that sort of intensity. I’m not sure he always makes it though.
It’s probably a lot easier to maintain that kind of intensity in a supporting role than in a leading role, so he’s at a disadvantage much of the time.
John Malkovich?
I thought of another one; Robert DeNiro. He apparently doesn’t do anything but intensity.
Toshiro Mifune.
Al Pacino
James Cagney
And what about Robert Mitchum?
Daniel Day Lewis
Crispin Glover
Charlton Heston
No kidding. He’s unfucking believable!
Minor NSFW link below. Contains “f” word. 
Viggo Mortensen
Dennis Hopper comes immediately to mind. Whether it’s Apocalypse Now or Blue Velvet, you get something really intense.
Everybody from Apocalypse Now put in a great performance, but in terms of intensity the following other actors from that film (which almost defines intensity): Martin Sheen; Robert Duvall; and of course Marlon Brando.
James Woods was the name that first popped to mind when reading the quiz question.
Gary Oldman has that quality and he’s played some superb villains.
I think Martin Sheen has a certain amount of intensity.
He was pretty laid back and understated in Jackie Brown.
TV Tropes has this covered.