Set pieces: it can work the other way too. Having a guy who is poor at CKs taking the CKs can have a spiralling effect or performance.
A big mistake by a player or several small mistakes can have a cumulative negative effect on future performance.
I do seem to get quite a few PKs and therefore they are a source of a few goals but the impact of a PK miss on my striker is too great to risk so, since my GK is least active player this season he takes the PKs (and scored his first from his first yesterday).
Captain: I usually set my 'best' player as Captain. If he stopped being best player or performance went down I'd change.
General principle: if you set a player to fill a role and performance decreases then set another.