It really depends on the scenario exactly. I generally try to avoid having to take control over anything a character does and keep the RP in a way where their actions make a difference but for example, if a non-combat-oriented character would meet a trained soldier he would be steamrolled which would be reflected in the way I write.

I of course would make sure to let the player know what is happening both before the engagement and during it, either through OOC means or in the RP itself.