If I cast phantasmal force on an NPC to make him think a creature has jumped onto his back and is trying to strangle him, assuming he fails his initial roll, why would he use an action to make an Investigation check to examine the illusion?
Would it be that he suspects he is the target of an illusion? Would he know that investigating it would get rid of it?
Phantasmal force also takes place during the player's turn, so he still has his turn, but it seems like he would probably use his actual turn attempting to get rid of what's attacking him, or if it's say, a monster standing in front of him, he might attack it with his weapon. Would you count attacking or attempting to defeat it as an Investigation roll, or does it specifically have to be that he is investigating it?
I can't see many reasons why your average person would actually stop and study a monster. If it were an actual monster that jumped on his back, or an actual skeleton that was summoned to fight him, he wouldn't investigate, he'd just try to attack. So I'm a bit confused about the Investigation part, unless one of his friends shouts 'the monster isn't real, look closely', but then they'd have to know he was fighting a monster and not just confused.
Is there a reason to investigate illusions like phantasmal force other than some kind of metagaming?
For the sake of roleplay, is it considered typical/expected for anyone affected by such an illusion to simply make an Investigation check each turn to attempt to end the illusion? Or is it considered normal for them to suffer during the player's turn a monster on their back trying to strangle them, and on their own turn, perform some unrelated action like attacking a player with their sword while ignoring the effect?