What actions, bonus actions or reactions can a character perform out of / outside of combat?

Question

DnD 5e has a fairly clear set of rules governing a player's possible Actions in Combat, and in other fast-paced situations a GM may break time into rounds. Among the listed actions, some would seem to initiate combat (e.g., Attack) but others are routinely performed out of combat (e.g., casting a ritual spell).

Certain rules make the combat actions sound like they are only for the purpose of managing fast-paced events. On the other hand, there are inconsistenties with this view. For example, spells cast as bonus actions are especially fast spells (citation needed), but cannot be cast as an action (citation needed), so length of time cannot be the only distinguishing characteristic between spells which can be cast as an action or bonus action.

So ... what can a player do in combat that they cannot do while adventuring?

Here's the start of an answer (to which I'll add as answers come in):

1. Behaviors that trigger combat. (Debatable, since the behavior may have actually occurred out of combat.)
2. Certain reactions whose triggers cannot occur out of combat.

Please clearly specify your reasons for your answer (e.g., RAW, RAI, logical deduction, common practice for groups you've been part of, etc). Citation of rules or expert opinion adds value to your answers.

Answer 1

Similar to how you can take an action outside of combat, you can also take bonus actions (assuming of course, they are available).

There are some noteworthy exceptions, specifically that if the bonus action has requirements they must be met. For instance, you could only parry if someone was attacking you. If you were casting a spell as a bonus action you would need the spell components, etc.

A good way to think about it - combat is just a special set of rules to help govern how combat takes place, and makes it easier for the DM to track everything happening in such a short period of time. Outside of needing to track time sensitive events there is no restrictions other then the ones mentioned above. I believe the related question & answer summarize this already.

Answer 2

You can take all actions that do not involve fighting outside of combat

The answer is essentially the same in all of these cases: combat is just a construct to help gameplay zoom in when a lot of things are happening at the same time. The PHB expains this on page 189:

Combat in D&D can be chaotic, deadly, and thrilling. This chapter provides the rules you need for your characters and monsters to engage in combat. (...) The game organizes the chaos of combat into a cycle of rounds and turns.

There is the proof by reductio ad absurdum: if you could not take actions outside of combat, outside combat you could not cast cure wounds or any other spell. Or drink a potion. Or remove something from your backpack. Or Run. Or Hide. Or Search for secret doors. Or help someone. And so on.

So in general, you can do anything you can do in combat also out of combat. Except for actual fighting.

Some actions make no sense without being in combat. Sure you can attack an object, e.g. to chop down a door outside combat. But if you want to attack a creature that could hit back, this implies combat. Parry only happens when you are being attacked and implies combat. Disengage only makes sense in combat. And there is a whole separate question on when combat actually starts, related to the Ready action.