Start of battle; enemy spies deploying; Friendly Fire.
"Erm John, you didn't have a moustache this morning..."
Suspicious is an unlockable combinative Ability acquired after unlocking the Skill of the same name. It negates any chances of Sabotage or Friendly Fire, additionally granting 200 Initiative per enemy Spy deployed. A very handy pick, considering that it (think about it) practically counters one and potentially counters one more of eight available Specializations, and not just any replaceables, but two quite unique and popular ones. If that's not good enough, he can't be shot in the back by his squad-mates during close quarters such as Extermination missions usually are.
Initiative points are granted as soon as any enemy spies deploy, giving first strike capability surpassed only by level 20+ spies (with no upgrades). Friendlies can still suffer from friendly fire themselves if the weapon is picked wrong.
Oddly, suspicious even seems to be available to Spies.
Note: The skill triggers when an enemy spy deployes together with the Suspicious trooper. Any survived enemy spy deployed on previous wave will not activate the skill when Suspicious trooper comes in new wave. The spy also needs to be deployed on the enemy side for the bonus initiative to take effect.