@26: On the other hand, I agree that if the character isn't "suspiciously similar" in some way then they aren't the trope. By that I am including their ensemble role as being similar. Just being a replacement character, one leaves and the other shows up, doesn't automatically make it this trope. For example, Doyle from Angel is killed mid-season one and the following episode Wesley (a secondary character from Buffy) shows up. He keeps the cast number (and gender ratio) the same but for all intents and purposes Doyle's role of having visions gets transferred to Cordelia, Wesley takes on a different job altogether.
And contrast that with Star Carrier's Trevor Gray and Donald Gregory. In the fourth book Gray's actually still one of the main viewpoint characters, but he's gotten a Rank Up and is now in command of The Battlestar instead of flying a Space Fighter. His "job" of being the pilot who gets looked down on by his squadmates for his background and uses his fighter's standard equipment in nonstandard ways to win battles gets transferred to new character Gregory.
So, I would argue that it's even possible to have an SSS when the first character is technically still around if they're no longer being used in their old role. (Probably a rare case though.)
Trust me, I'm an engineer!

Bumping after some time...
The primary thing about this trope is that replacement characters do tend to resemble who they replace to some degree, whether it be personality or ensemble role. We've learned from experience that trying to define/distinguish tropes by putting a gauge on it rarely works, in this case the question is how much can they resemble the previous character before they are no longer "suspiciously similar." In comparison there was a TRS on Sacrificial Lamb and Sacrificial Lion where we had to distinguish the tropes beyond "x amount of screentime" (coming to the conclusion it is about how significant the death plays out on the plot and affects the other characters, the Lion tends to be a Wham Episode).
On the other hand, I agree that if the character isn't "suspiciously similar" in some way then they aren't the trope. By that I am including their ensemble role as being similar. Just being a replacement character, one leaves and the other shows up, doesn't automatically make it this trope. For example, Doyle from Angel is killed mid-season one and the following episode Wesley (a secondary character from Buffy) shows up. He keeps the cast number (and gender ratio) the same but for all intents and purposes Doyle's role of having visions gets transferred to Cordelia, Wesley takes on a different job altogether.
edited 21st Feb '14 10:56:36 PM by KJMackley