Most skill cards have an ability that triggers if you win the test. What happens if another investigator in your location uses a skill card to aid your test? Which player gets the benefit? Drawing a card seems ambiguous about who draws it. For Deduction, it would seem to imply that the player doing the test gets the extra clue. Opportunist, on the other hand, seems to imply that it goes back to the hand of the player who played the card. The only one that is explicit is Survival Instinct, where it specifies 'the evading player'.

Opportunist cannot be committed to other players tests.

As for others, as they refer to "you", I read them as if they were speaking to the one who committed the cards, not the one resolving the test, so if you commits Guts to someone else's test and that someone succeeds, you draw 1 card, not him.

The official response from someone asking this at Arkham Nights to Matt Newman is that skill card trigger benefits (like the card draw from Guts) go to the owner of the card, unless the skill card changes the outcome of the test (as opposed to just the odds of that outcome happening). Most obviously, this makes Vicious Blow playable by allies (otherwise they're not boosting any attack), but this also means the person doing the investigating gets the bonus clue from Deduction.

Good point about Opportunist. However, after looking at the cards, they don't have the word 'you' in them. For example, Fealess says the following " If this skill test is successful, heal 1 horror." It never specifies who heals the horror. I'm inclined to agree that it applies to the character who played the card, but the wording on Deduction is " If this skill test is successful while investigating a location, discover 1 additional clue at that location." The use of the word 'additional' would seem to imply that it applies to the character performing the test.

Well, what I meant, the cards doesn't directly says "you", but it says as it was adressing you, not, for example, "if this skill test is successful, investigator performing the test heals 1 horror".

A good way to think about it is that if it affects the test directly (doing an extra damage, discovering an additional clue) it happens to the investigator doing the test. If it's a side benefit (heal 1 horror, draw a card) it happens to the player playing the card. You own the card, so the effect happens to you in those instances, but if it specifically modifies the results of the success, it goes to the player doing the test.