To use the example of Gimli, he's part of the Fellowship, and nearly always appears alongside the rest in a Tenth Walker fic. It's the fact that he remains (usually, mostly) himself despite being touched by the badfic's words--and the fact that happened consistently through dozens and dozens of LotR missions--that make him "OOC Resistant."

Personally, I feel a character should show up in numerous missions, consistently remaining in-character in the face of the author's Words, before we tack the OOC resistance label on them.

IIRC OOC Resistance is a special trait. Like 99 said, characters ignored by the Suethor will be in-character because there's nothing to drive them out of it; to take an example from my recent mission, Bruenor Battlehammer, Catti-brie, Jarlaxle and Artemis Entereri are all IC despite being from the Drizzt series because they don't appear in that particular "fic" I sporked; I don't think any of the above characters merit OOC Resistance, which is usually reserved to powerful beings such as Elrond and the Valar or consistently-ignored characters such as Gaspode the Wonder Dog, the TARDIS or Gimly.

I'd imagine characters who don't receive attention from the Sue (positive or negative) would be in character.

THE EMPEROR PROTECTS!

I don't think so. The reason that some characters are "immune" to the Suefluence seems to be more attributed to a lack of positive attention by Suethors rather than the character's own volition or will. Likewise, many characters seem immune to OOC-ness due to a lack of attention.
This is just my observation, anyways :0