For me he’s a great example of what the OP asks - I have the hots for him but only as the character Dr House. I’d watched him as an actor/comedian for years and years and he never remotely registered as fanciable. Then he’s starts playing a miserable tortured sarcastic genius and I swoon. I do wonder what this says about my taste in men.
Firestar from Spider-Man and his Amazing Friends. She was absolutely undeniably hot, pun intended. She was introduced in X-Men comic a few years later and got her own miniseries but she was no way as intriguing there as she was in the cartoon.
Benedict Cumberbatch is gorgeous in anything he does - Jesus Christ, those cerulean eyes, that velvet voice - but OMFG him as Sherlock. “Brainy is the new sexy” indeed!
Oddly enough, Martin Freeman is the opposite of this for me - I don’t find myself attracted to him at all when watching him on screen, but his personality in interviews makes me want to strip him naked and have his way with me IRL.
BSG had plenty of eye candy. Tricia Helfer as “Six” was practically sex incarnate. But these two were the instant boner-inducers for me: Kandyse McClure (“Dee”) - my god, those eyes Katee Sackhoff (“Starbuck”) - I know some viewers didn’t like her much, but she totally turns me on. She’s on *Longmire *now.
Have you seen him blonde in the new Julian Assange movie? I think he’s super sexy with his natural hair color, but have kind of a refusal for the near albino look.
Cobie Smulders in The Avengers. I liked her in early HIMYM as well, but in recent years her character has got more annoying. While in Avengers she is just kick-ass and hot and wears a military uniform, thank you very much. Apparently she cameos in the new Agents of SHIELD.
Oh yes, I’d almost forgotten about her! In the very same movie that had Scarlett Johansen looking like every man’s dream, Cobie about smacked me out of my theater seat when I first saw her.
Another one I need to add: Holliday Grainger in her guest appearance as Sophia, a “villain” in a first season episode of the BBC’s “The Adventures of Merlin”. Again, the photo doesn’t convey it, but in the TV episode she just got me all wound up. She didn’t do anything overtly “sexy”, and she spent the entire episode in an ankle-length dress. Instead, it was all quick, sideways glances at Arthur, and it just came across in a way that I couldn’t resist.
Anyway, I wanted to re-emphasize the intent of my OP. Most posters have gotten it, but there has been a bit of drift. I’m not looking for actors/actresses you simply find “hot”. I’m looking for those cases where the instant a character appears onscreen, you go “WHOA! ME WANT!” In my own personal examples, I find it interesting that the actresses in question have been those I probably wouldn’t go out of my way to find more of their work. They just played a particular part, maybe in a particular scene, where there was something about it that pushed all the right buttons for me.