There is a part of an RPG which is "let the rules and dice tell you how it resolved." Which is to say, if the player wants to do something, then you say "Yes" and let them interact with the rule that guides them on how to do it. (Skill rolls, attack rolls, etc)
Then there is this other part which is Narrative play. This comes into play with The Fiction. And that is "all the things described by the GM in the scene."
Meaning that a player assuming an outdoor attack on a roadside might have a hill side to the path which has rocks the orcs can slip and fall on = seems reasonable, even if the GM never mentioned a hill or rocks, it fits The Fiction and so there is no reason to say no.
The fear then comes in with the halfling saying "I fly up the hill", and there is no Fiction to indicate the halfling can fly in any way. I think this is actually a straw man argument. Personally, I have never ever had a player, so egregiously, dictate their character doing something that has no reason or rule or ability to back it up.